HIGH  TEMPERATURE 
GAS  THERMOMETRY 


BY 

ARTHUR  L.  DAY  AND  ROBERT  B.  SOSMAN 

WITH 

AN  INVESTIGATION  OF  THE  METALS 
BY  E.  T.  ALLEN 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
1911 

^  <3  rf  1  r; 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  157 


2611 


PRESS  OF  GIBSON  BROTHERS 
WASHINGTON,  D.  C. 


Physics 
Library 


TABLE  OF  CONTENTS. 


Page. 

Introduction v 

1 .  Temperature  definition i 

2.  Historical 4 

3.  Importance  of  absolute  measurements  above  1 150° 12 

4.  The  experimental  problem  in  gas  thermometry 14 

5 .  Apparatus 17 

Furnace 17 

Manometer 19 

Unheated  space 20 

Barometer 22 

Thermo-electric  apparatus 23 

Bulb 24 

6.  Plan  of  procedure 25 

7.  Expansion  coefficient  of  platin-iridium  bulb 27 

Apparatus  and  method 27 

Experimental  data 36 

8.  Pressure  coefficient  of  nitrogen 40 

9.  Gas-thermometer  measurements.     First  series 40 

Computation  of  results 40 

Experimental  data 41 

Melting-points  based  upon  the  first  series 46 

10.  Introduction  to  the  second  series,  including  the  higher  temperatures 48 

1 1 .  Changes  in  the  apparatus 5° 

The  platin-rhodium  bulb 50 

The  furnace 5" 

12.  Details,  errors,  and  corrections 52 

Temperature  of  gas  in  bulb 54 

Definition  of  temperature  by  measurement  of  pressure 57 

The  gas 60 

Expansion  coefficient  of  the  bulb 61 

Thermo-electric  measurements 63 

Summary  of  errors 69 

13.  Experimental  data  and  calculated  results 7° 

Expansion  coefficient  of  platin-rhodium 7° 

Gas- thermometer  data 72 

14.  The  transfer  to  the  fixed  points 79 

15.  The  metals  used.     By  E.  T.  Allen 85 

1 6.  The  fixed  points 97 

Further  details  on  the  substances  employed  for  temperature  constants ...  97 

Melting-point  measurements 9$ 

Temperature  of  the  fixed  points 100 

17.  Interpolation  between  the  fixed  points 109 

1 8.  Extrapolation  upward.     The  meltmg-pojint  of  platinum 113 

19.  The  thermo-element  curve  from  o°  to  1755° 1 16 

20.  Relation  of  thermal  E.  M.  F.  to  composition 120 

2 1 .  Summary 124 

in 


DAY  AND  SOSMAN 


FRONTISPIECE 


INTRODUCTION. 

The  year  1900  marked  the  close  of  an  investigation  of  the  gas  thermometer 
at  high  temperatures  which  had  been  prosecuted  with  great  vigor  at  the 
Reichsanstalt  for  nearly  ten  years.  The  temperature  scale  had  not  been 
extended  beyond  1 150°  C.,the  stopping-place  of  most  of  the  earlier  investi- 
gations, but  a  scale  had  been  established  in  which  there  was  a  strong  feeling 
of  confidence,  and  which  is  to-day  in  general  use.  The  absolute  accuracy 
was  believed  to  be  about  i°  at  400°  and  2°  to  3°  at  1150°.  Temperatures 
higher  than  this  were  extrapolated  by  thermo-electric  or  radiation  methods, 
at  first  with  confidence,  afterward  with  some  misgiving,  concerning  which 
a  good  deal  is  said  in  the  pages  which  follow.  This  scale  and  extrapola- 
tion scheme  have  never  been  formally  adopted  by  international  agreement, 
although  no  doubt  such  an  agreement  would  have  been  of  considerable 
value.  In  the  absence  of  prescribed  procedure  the  details  of  the  extrapola- 
tion have  differed  considerably  in  the  hands  of  different  experimenters, 
which  has  caused  an  uncertainty  in  the  interpretation  of  the  higher  tem- 
peratures, amounting  to  50°  at  the  platinum  point.  It  was  primarily  for 
the  purpose  of  clearing  up  this  uncertainty  in  the  region  above  1 1 50°  and 
providing  a  sound  basis  of  temperatures  for  the  mineral  work  of  this  labora- 
tory that  this  problem  was  taken  up  again. 

The  present  investigation  was  begun  in  1904  under  the  auspices  of  the 
U.  S.  Geological  Survey  and  on  the  fourth  floor  of  its  building  in  the  busiest 
portion  of  Washington.  Here  the  conditions  of  light,  constant  temperature, 
and  stability  were  very  ill-adapted  for  quantitative  work  of  the  high  order 
of  accuracy  to  which  it  was  desired  to  attain.  It  was  therefore  most  fortu- 
nate for  the  success  of  this  and  other  quantitative  work  which  had  hitherto 
been  carried  on  with  considerable  difficulty,  when  the  Carnegie  Institution 
of  Washington  provided  a  new  and  well-equipped  laboratory  for  it  in  1907. 
The  preliminary  experimental  work  and  all  the  measurements  of  the  first 
series  here  described  (to  1150°)  were  made  in  the  Geological  Survey;  those 
of  the  second  series,  including  all  the  observations  of  temperatures  above 
1 1 50°,  were  made  in  the  Geophysical  Laboratory.  The  facilities  provided 
in  the  new  laboratory  permitted  measurements  of  great  refinement. 

Dr.  J.  K.  Clement,  who  shared  the  burden  of  observations  under  the 
unfavorable  conditions  of  the  Survey  building,  withdrew  in  the  spring  of 
1907  to  become  physicist  of  the  Technologic  Branch  of  the  Survey  (now 
the  Bureau  of  Mines).  The  determination  of  the  expansion  coefficient  of 
the  platin-iridium  bulb  (p.  27),  and  all  the  work  with  the  platin-rhodium 
bulb  (p.  48  et  seq.)  was  done  by  Day  and  Sosman. 

If  it  is  permitted  to  a  student  who  has  been  actively  at  work  upon  this 
problem  of  high  temperature  measurement;  partly  in  the  Reichsanstalt  and 
partly  independently,  for  a  nearly  continuous  period  of  fourteen  years,  to 
indulge  in  speculation  of  a  somewhat  irresponsible  kind,  this  may  be  said 
apropos  of  the  present  status  of  absolute  measurements  of  high  temperature : 
The  gas  thermometer  with  a  bulb  of  platin-rhodium  is  thoroughly  trust- 


VI  INTRODUCTION. 

worthy  and  free  from  the  large  and  more  or  less  indeterminate  errors  char- 
acteristic of  much  of  the  earlier  work,  but  the  upper  limit  of  temperature 
attainable  with  this  bulb  is  nearly,  if  not  quite,  reached  at  1550°.  The 
platin-rhodium  alloy  is  not  rigid  enough  to  be  depended  upon  for  constant- 
volume  measurements  at  higher  temperatures.  The  iridium  bulb  in  use  at 
the  Reichsanstalt  has  the  required  stiffness,  but  can  not  be  used  for  the 
calibration  of  thermo-elements  at  temperatures  above  1550°  without  con- 
taminating them  beyond  the  possibility  of  restoration  for  accurate  work. 
It  may  prove  practicable  to  coat  the  surface  of  the  bulb  or  the  exposed 
wire  with  a  viscous  silicate  or  oxide  which  will  prevent  the  sublimation  of 
iridium,  but  all  the  efforts  thus  far  made  to  find  such  a  substance  have  been 
without  success.  And  even  if  this  proved  successful  it  is  unlikely  that  it 
would  add  more  than  100  or  200  degrees  to  the  existing  scale. 

A  better  plan  might  be  to  abandon  platinum  metals  altogether  in  the 
near  neighborhood  of  their  melting  temperature  and  to  substitute  tungsten 
or  tantalum  in  a  bath  of  stable  oxides  or  silicates  of  sufficient  fluidity  to 
permit  it  to  be  stirred.  Such  a  bath  could  perhaps  be  heated  by  passing 
the  current  directly  through  it,  as  in  the  barium  chloride  bath  introduced 
by  the  General  Electric  Company  a  few  years  ago.  Water- jacketing  would 
protect  the  containing  vessel,  and  a  stirring  mechanism  and  diminished 
pressure  above  the  surface  of  the  liquid,  with  a  bulb  of  appropriate  shape, 
would  make  it  entirely  practicable  to  maintain  a  nearly  constant  pressure 
inside  and  outside  of  the  bulb.  The  bath  would  protect  the  bulb  from  oxida- 
tion and  the  stirring  would  provide  a  constant  temperature  about  it — which 
is  the  greatest  uncertainty  in  the  present  system  of  gas-thermometer  meas- 
urement. All  depends  upon  discovering  a  bath  which  will  meet  these 
difficult  conditions.  In  a  field  which  is  still  quite  unexplored  perhaps  we 
need  not  altogether  despair  of  finding  it. 

Meanwhile  accurate  gas  thermometry  up  to  1550°  insures  the  accurate 
calibration  of  optical  and  radiation  pyrometers,  which  are  well  founded 
theoretically,  convenient  to  use,  and  without  an  upper  temperature  limit. 
There  is  no  need  that  the  measuring  instrument  be  in  contact  with  the  hot 
body  whose  temperature  is  desired,  nor  is  it  always  necessary  that  any 
portion  of  the  apparatus  be  heated  to  this  temperature.  The  sensitiveness 
of  most  of  these  instruments  is  relatively  low,  but  at  extreme  temperatures 
it  is  quite  sufficient  for  industrial  and  probably  for  scientific  uses  also  for 
many  years  to  come.  It  would  therefore  appear  that  we  are  now  well  served 
in  the  matter  of  accurate  and  trustworthy  high-temperature  standards  and 
convenient  measuring  devices,  and  if  the  need  for  a  more  extended  and 
accurate  scale  shall  arise  it  is  by  no  means  certain  that  the  limit  of  standard 
temperature  definition  by  means  of  the  gas  scale  has  been  reached  at  1550°. 


HIGH  TEMPERATURE  GAS  THERMOMETRY. 


I.  TEMPERATURE  DEFINITION. 

The  measurement  of  temperature  differs  from  most  fundamental  physical 
measurements  in  that  the  temperature  function  is  not  additive.  There  is 
no  temperature  unit  corresponding  to  a  foot-rule  or  meter-stick  which  can 
be  applied  successively  to  measure  a  high  temperature  as  we  would  measure 
the  height  of  a  room.  Two  temperatures  of  one  degree  can  not  be  combined 
in  any  way  to  give  a  temperature  of  two  degrees.  Temperature  measure- 
ment is  therefore  wholly  a  matter  of  arbitrary  definition,  of  selecting  some 
convenient  phenomenon  (like  the  expansion  of  a  gas)  which  varies  continu- 
ously and  as  nearly  as  possible  uniformly  with  temperature  changes,  of 
providing  convenient  arbitrary  units  of  subdivision,  and  then  of  observing 
the  expansion  of  the  gas,  or  other  phenomenon,  under  the  conditions  which 
surround  the  unknown  body  whose  temperature  is  desired.  The  expansion 
of  hydrogen  has  been  established  by  international  agreement  as  our  funda- 
mental measure  of  temperature.  The  gas  thermometer  is  therefore  now  the 
standard  thermometer  in  terms  of  which  all  temperatures  are  denned. 

The  theoretical  interest  in  gas  thermometry  centers  about  the  quantita- 
tive relation  existing  between  the  increase  in  the  temperature  of  the  gas 
expanding  under  constant  pressure  or  volume  and  the  quantity  of  heat 
required  to  produce  it.  It  is  a  somewhat  inaccessible  question  by  reason 
of  the  difficulty  of  approaching  it  experimentally  with  the  required  accuracy. 
Although  the  amount  of  the  expansion  of  gases  is  more  closely  proportional 
to  the  quantity  of  heat  applied  than  that  of  liquids  or  solids,  no  strictly 
"perfect  gas"  in  this  sense  has  been  found.  The  amount  of  the  divergence 
between  the  expansion  and  the  quantity  of  heat  which  produces  it  is 
slightly  different  with  different  gases,  and  is  also  slightly  different  for  the 
same  gas  at  different  temperatures.  In  the  case  of  nitrogen,  which  has  a 
greater  range  of  practical  utility  than  other  gases  thus  far  studied,  the 
expansion  curve  diverges  slowly  from  the  regular  curve  of  a  perfect  gas  as 
the  temperature  increases,  but  the  amount  of  its  departure  does  not  attain 
the  magnitude  of  one  degree  centigrade  until  the  temperature  reaches  1 100° 
or  more,  and  the  magnitude  of  this  correction  factor,  if  it  is  to  be  regarded 
as  a  correction  factor,  is  probably  of  the  same  order  as  the  observation 
errors  of  an  actual  gas  thermometer  in  the  present  stage  of  its  development. 
For  this  reason,  these  theoretical  considerations,  which  have  been  admirably 
treated  by  Buckingham  in  a  paper,  "On  the  establishment  of  the  thermo- 
dynamic  scale  of  temperature  by  means  of  the  constant-pressure  ther- 
mometer" (Bull.  Bur.  Standards  3,  237,  1907),  do  not  seriously  affect  the 
definition  of  an  accurate  and  practicable  high-temperature  scale  by  means 
of  the  gas  laws. 


2  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

Buckingham  defines  the  gas  scales  very  clearly  in  this  way: 

'  '  Two  principal  methods  of  gas  thermometry  are  in  use.  In  the  constant- 
volume  thermometer  a  mass  of  gas  is  kept  at  constant  volume  and  its 
pressure  observed  at  the  melting-point  of  ice,  at  the  condensing-point 
of  steam  (under  standard  atmospheric  pressure),  and  at  the  temperature 
to  be  determined.  If  these  three  pressures  be  denoted  by  p0,  ploo,  and  p, 
the  centigrade  temperature  according  to  the  scale  of  this  thermometer 
is  by  definition, 


The  numerical  value  thus  assigned  to  a  given  temperature  depends  slightly 
on  the  initial  pressure  and  is  somewhat  different  for  different  gases.  *  *  * 
"In  the  constant-pressure  thermometer  a  mass  of  gas  is  kept  at  constant 
pressure,  and  its  volume  observed  at  the  two  standard  temperatures  and 
at  the  temperature  to  which  a  numerical  value  is  to  be  assigned.  If  these 
volumes  be  v0,  vloo,  and  v,  the  centigrade  temperature  according  to  the 
scale  of  this  thermometer  is  by  definition 

v  —  v0 


The  resulting  value  depends  somewhat  on  the  magr'  .ude  of  the  constant 
pressure  and  on  the  nature  of  the  gas  used." 

Lord  Kelvin  has  proposed,  and  the  physicists  who  choose  to  consider  the 
problem  from  the  theoretical  side  (including  Buckingham)  have  generally 
approved  the  proposal,  to  go  further  and  interpret  all  temperatures  strictly 
in  terms  of  a  hypothetical  "perfect  gas,"  in  which  the  expansion  (which 
defines  the  temperature),  whether  under  constant  volume  or  pressure,  would 
increase  exactly  in  proportion  to  the  quantity  of  heat  required  to  produce 
it.  This  would  have  the  obvious  advantage  that  temperature  definition 
would  become  uniform  and  independent  of  the  properties  of  any  particular 
substance,  but  its  adoption  will  be  of  little  actual  service  to  high-tempera- 
ture thermometry  until  experimental  measurements  of  greater  scope  and 
precision  are  available. 

The  direct  experimental  measurement  of  the  degree  of  departure  of  an 
actual  gas  from  this  ideal  state  (the  Joule-Thomson  porous-plug  experi- 
ment) involves  the  determination  of  two  extremely  small  magnitudes  which 
have  not  (as  yet)  been  measured  separately  at  all,  nor  together  except  with 
the  greatest  difficulty.  From  such  results  as  we  have  it  appears,  as  has 
been  stated,  that  the  difference  between  the  expansion  of  the  thermo- 
dynamically  perfect  gas  and  the  actual  expansion,  from  the  low  initial 
pressures  usually  used,  of  any  of  the  gases  hitherto  employed  for  the  pur- 
pose (H2,  N2,  CO,  CO2,  O2)  approaches  i°  at  1100°.  Even  for  work  of  the 
highest  precision,  the  constant-pressure  or  the  constant-volume  gas  scale 
is  therefore  quite  as  serviceable  as  the  thermodynamic  scale,  at  the  present 
stage  of  development  of  the  subject. 

By  international  agreement  and  nearly  universal  practice,  two  fixed  tem- 
peratures are  accepted  as  the  basis  of  the  modern  temperature  scale:  The 
melting-point  of  pure  ice  and  the  boiling-point  of  pure  water,  both  at  normal 
atmospheric  pressure  (equal  to  760  mm.  of  mercury).  The  interval  included 


TEMPERATURE   DEFINITION.  3 

between  these  two  temperatures  is  subdivided  into  zoo  parts  by  measuring 
the  constant  volume  expansion  of  hydrogen  from  an  initial  pressure  of  i  m. 
of  mercury  throughout  this  interval. 

The  actual  determination  of  these  intervals  or  degrees  centigrade  was 
made  by  Chappuis  at  the  Bureau  Internationale  des  Poids  et  Mesures  at 
Paris  in  1888,  and  is  a  work  of  such  painstaking  character  in  most  particu- 
lars that  no  investigator  has  found  it  necessary  to  repeat  it  since  that  time. 
The  probable  accuracy  of  the  individual  degrees  thus  determined,  that  is, 
of  the  international  scale  of  temperatures  between  o°  and  100°,  is  stated  by 
Chappuis  to  be  0.002. ' 

To  extend  the  scale  beyond  these  limits  in  either  direction,  it  is  (theo- 
retically) only  necessary  to  continue  the  measurement  of  the  expansion  of 
hydrogen  under  the  same  conditions  down  to  or  up  to  any  desired  tempera- 
ture. In  practice,  this  procedure  encounters  various  experimental  difficulties 
which  increase  rather  rapidly  as  we  go  farther  away  from  the  temperatures 
of  e very-day  life. 

These  experimental  difficulties  serve  to  place  definite  limits  upon  the 
accuracy  of  temperature  measurement  at  present  attainable  in  different 
parts  of  the  field,  of  which  it  is  important  to  obtain  a  clear  idea  in  order 
to  be  able  to  form  a  sound  judgment  of  the  probable  significance  of  measure- 
ments at  widely  different  temperatures.  If  we  are  told,  for  example,  that 
a  solution  boils  at  90.27°,  we  recognize  that  this  accuracy  is  entirely  practi- 
cable. If,  on  the  other  hand,  platinum  is  said  to  melt  at  1755.46°,  it  is  of 
advantage  to  know  that  the  last  two  figures  are  wholly  without  significance, 
and  the  fourth  is  uncertain. 

There  is  no  denying  the  fact  that  a  great  deal  still  remains  to  be  done 
upon  the  experimental  side  before  the  steadily  advancing  requirements  of 
both  science  and  industry  in  the  matter  of  a  trustworthy  temperature  scale, 
of  sufficient  accuracy  and  more  particularly  of  sufficient  range,  can  be 
satisfied.  It  is  no  disparagement  of  the  present  system  of  temperature 
definition  to  say  that  the  gas  thermometer  itself  is  a  complicated  and 
cumbersome  instrument  to  use  in  any  of  the  forms  which  have  hitherto 
been  devised,  and  possesses  limitations,  both  of  range  and  accuracy,  which 
are  difficult  to  overcome. 

One  consequence  of  this,  particularly  in  the  region  of  high-temperature 
measurements,  is  that  temperatures  easily  come  to  be  regarded  with  unwar- 
ranted confidence  in  the  hands  of  those  who  have  never  acquired  a  first-hand 
knowledge  of  these  limiting  conditions.  This  confidence  has  no  doubt  been 
fostered  by  the  comparative  ease  with  which  relative  measurements  of  tem- 
perature can  be  made,  even  in  the  more  inaccessible  parts  of  the  scale,  with 
the  thermo-element  and  the  resistance  thermometer.  These  devices  are 
sensitive  to  temperature  differences  of  the  order  of  magnitude  of  0.01° 
throughout  their  entire  range,  but  they  are  dependent  absolutely  upon 
fundamental  measurements  with  the  gas  thermometer  for  their  evaluation 
in  terms  of  the  generally  accepted  degree  centigrade. 

'In  a  recent  paper,  "Some  new  measurements  with  the  gas  thermometer"  (Amer.  Jour.  Sci.  (4),  26, 
405-463,  1908).  the  authors  gave  it  as  their  opinion  that  the  determination  of  the  expansion  coefficient  of 
the  thermometer  bulb  used  by  Chappuis.  which  is  of  course  an  important  factor  in  the  experimental  problem, 
would  not  warrant  an  assumption  of  accuracy  greater  than  0.005°.  To  this  Chappuis  replied  in  a  personal 
letter  (1909^  that  considering  all  the  factors  of  the  problem,  and  subsequent  experience,  he  still  thought 
0.002°  a  fair  estimate  of  the  probable  error  of  the  scale  in  the  region  between  o°  and  100°. 


4  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

It  is  sufficiently  obvious,  though  often  carelessly  overlooked,  that  no 
method  of  temperature  measurement,  however  sensitive  or  adaptable  it  may 
be,  can  yield  temperatures  of  greater  absolute  accuracy  than  the  system  in 
terms  of  which  those  temperatures  are  defined.  With  the  gas  thermometer 
as  our  basis  of  definition,  therefore,  we  shall  know  our  temperatures  with 
just  the  certainty  which  it  is  able  to  furnish  and  no  more.  There  is,  to  be 
sure,  some  justification  for  expressing  the  results  of  thermo-electric  or  resist- 
ance measurements  in  units  smaller  than  the  errors  of  the  fundamental 
scale,  where  only  comparative  measurement  is  involved ;  but  such  measure- 
ments must  not  be  used  without  regard  to  this  limitation. 

It  is  not  the  purpose  of  the  present  paper  to  discuss  the  general  problem 
of  thermometry,  or  the  particular  advantages  of  one  system  of  thermometric 
measurement  over  another  in  laboratory  or  industrial  practice,  but  merely  to 
describe  the  work  which  has  been  done  in  recent  years  to  increase  the  range 
and  accuracy  of  the  temperature  scale,  upon  which  the  various  devices 
for  measuring  high  temperatures  depend  for  their  calibration.  Neither  is 
it  necessary  to  include  any  discussion  of  the  relative  theoretical  merit  or 
experimental  practicability  of  the  constant-volume  and  constant-pressure 
methods  of  gas  thermometry.  The  reader  will  find  it  in  Buckingham's 
paper  already  mentioned,  in  Barus's  review1  of  the  progress  of  pyrometry 
for  the  Paris  Congress  in  1900,  as  well  as  in  earlier  discussions  to  which 
they  have  referred.  The  choice  of  the  one  or  the  other  method  is  likely  to 
be  governed  by  the  taste  or  predilection  rather  than  by  the  necessities  of 
the  individual  experimenter.  Neither  system  possesses  decided  advantages 
over  the  other.  Most  of  the  recent  measurements  have  been  made  with  the 
constant-volume  system. 

2.  HISTORICAL. 

To  reach  a  satisfactory  estimate  of  the  degree  of  confidence  to  be  accorded 
to  the  new  absolute  measurements  of  high  temperature  requires  some 
historical  perspective,  for  the  development  of  the  experimental  problem 
has  now  been  going  on  for  nearly  a  century.  It  will  therefore  be  well  to 
review  somewhat  briefly  the  chief  steps  in  its  progress  for  two  explicit 
reasons:  (i)  In  order  that  we  may  form  a  sound  and  properly  critical 
judgment  of  the  trustworthiness  of  the  results  so  far  attained;  and  (2) 
that  future  investigators  may  be  enabled  to  avoid  the  limitations  which 
have  arisen  in  the  earlier  work. 

We  will  therefore  attempt  to  follow  the  development  of  the  gas  ther- 
mometer through  some  of  the  vicissitudes  of  its  long  service  as  a  standard 
high-temperature  measuring  instrument.  For  it  was  understood  from  the 
beginning  that  the  gas  thermometer,  with  its  cumbersome  bulb  and  equip- 
ment, must  always  remain  an  instrument  of  reference  rather  than  of  prac- 
tical utility. 

Prinsep,  1828. — The  first  pyrometer  based  on  the  expansion  of  gases, 
so  far  as  we  now  know,  was  made  by  Prinsep2  and  described  by  him  in 
1828.  He  used  a  bulb  of  gold,  connected  with  a  sensitive  manometer  with 

'Carl  Barus,  "Les  progres  de  la  pyrometrie,"  Rapports  presented  au  Congres  Internationale  de  Physique, 
1900,  vol.  i,  pp.  148-177. 

'Phil.  Trans.,  1828,  79-95;  Ann.  d.  chim.  et  d.  phys.  (2),  41,  p.  247,  1829. 


HISTORICAL.  5 

which  to  maintain  the  gas  (air)  at  constant  pressure  within,  and  connected 
also  with  a  reservoir  of  olive  oil ;  the  expansion  of  the  air  in  the  bulb  dis- 
placed a  proportionate  amount  of  oil,  which  was  caught  and  weighed  and 
the  temperature  calculated.  With  this  apparatus  Prinsep  made  excellent 
temperature  measurements,  chiefly  of  the  melting-points  of  the  alloys  of 
gold,  silver,  and  platinum,  which  bear  his  name  and  are  still  sometimes 
used.  The  usefulness  of  Prinsep's  thermometer  was  limited  by  the  com- 
paratively low  melting  temperature  of  the  gold  bulb. 

Pouillet,  1836. — Prinsep  was  quickly  followed  by  Sir  Humphrey  Davy 
and  several  others,  all  employing  the  expansion  of  air  at  constant  pressure, 
but  none  contributing  materially  to  the  improvement  of  Prinsep's  apparatus 
until  Pouillet1  constructed  his  instrument  in  1836.  Pouillet's  bulb  was  of 
platinum,  which  enabled  him  to  reach  the  highest  temperatures,  and  his 
experimental  procedure,  with  but  slight  modifications,  is  that  employed 
to-day  by  Callendar  and  his  associates,  who  have  always  expressed  a  prefer- 
ence for  the  constant-pressure  method.  It  was  Pouillet  also  who  made  and 
calibrated  the  first  practicable  thermo-element  (platinum-iron),  who 
anticipated  the  method  of  measuring  temperature  through  determinations 
of  the  specific  heat  of  platinum  subsequently  developed  by  Violle,  and  who 
made  some  study  of  the  radiant  energy  sent  out  by  glowing  solids.  In 
varying  degree  and  with  many  of  the  inevitable  limitations  of  the  pioneer, 
Pouillet  not  only  established  gas  thermometry  upon  a  sound  basis,  but 
introduced  several  of  the  important  practical  methods  of  pyrometry  (specific 
heat,  thermo-electricity,  radiation)  which  have  been  in  use  since  his  time. 

Following  Pouillet,  therefore,  the  advancement  of  pyrometric  measure- 
ment became  to  a  considerable  degree  a  question  of  perfection  of  experi- 
mental detail  rather  than  of  the  development  of  new  principles,  and  so, 
with  one  or  two  exceptions  which  will  be  noted  presently,  it  has  since 
remained.  Regnault  in  particular  made  a  number  of  improvements  in 
the  Pouillet  instrument  in  1847. 2 

The  first  gas  thermometer  which  measured  the  expansion  of  the  gas  under 
constant  volume  appears  to  have  been  built  by  Silbermann  and  Jacquelin 
in  1853,  but  it  was  only  indifferently  successful.  Effective  use  was  first 
made  of  the  method  in  the  Avork  of  Becquerel  described  below. 

St.  Claire-Deville  and  Troost,3  1857. — It  was  soon  after  this  that  a  real 
catastrophe  occurred  in  the  development  of  the  gas  thermometer.  Deville 
and  Troost  (1857),  desiring  to  use  a  heavier  gas  in  place  of  air,  introduced 
iodine  into  a  bulb  of  porcelain  and  made  determinations  of  a  number  of 
constant  temperatures,  most  conspicuous  among  which,  in  the  discussion 
which  followed,  was  the  boiling-point  of  zinc,  which  they  ascertained  to 
be  1040°. 

Edmond  Becquercl,*  1863. — Becquerel  followed  in  1863,  using  the  Pouillet 
apparatus  with  platinum  bulb  and  air  as  the  expanding  gas,  and  reached 
the  conclusion  that  zinc  boiled  at  932°,  more  than  100°  lower.  In  the  con- 
troversy which  followed,  and  which  was  maintained  from  both  sides  with 
considerable  bitterness,  these  observations  were  repeated  by  both  observers 

'Compt.  rend.,  3,  782-790,  1836. 

-Relation  des  Experiences,  Mem.  Acad.  Sci.,  Paris,  21,  p.  168,  1847. 

:1Deville  and  Troost;  Compt.  rend.  45,  821-825,  1857;  49,  239-242,  1859;  Ann.  d.  chitn.  et  d.  phys.  (3), 
58,  257-299,  1860. 

4Ann.  d.  chim.  et  d.  phys.  (3),  68,  49~i43.  1863. 


6  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

with  substantial  confirmation  of  the  first  results,  Deville  and  Troost  main- 
taining from  experiments  of  their  own  that  Becquerel's  platinum  bulb  was 
permeable  to  hot  gases  and  that  his  results  must  of  necessity  be  too  low. 
To  this  contention  Becquerel  replied  convincingly  by  using  a  porcelain  bulb 
himself  (still  retaining  air  as  the  expanding  gas),  with  both  the  constant- 
volume  and  constant-pressure  methods  of  measurement,  and  announced  a 
result  (891°)  even  lower  than  his  previous  determination.  Notwithstand- 
ing this,  Deville  and  Troost  were  unwilling  to  regard  the  result  as  con- 
clusive, and  looked  upon  the  discrepancy  between  Becquerel's  earlier  and 
later  results  (932°  and  891°)  with  unconcealed  suspicion.  They  reiterated 
their  belief  that  the  platinum  bulb  was  permeable  and  that  Becquerel's 
results  with  porcelain  bulbs  were  still  too  low  through  failure  to  expose  the 
bulb  directly  to  the  zinc  vapor.  Becquerel's  bulb  had  been  shielded  from 
the  direct  action  of  the  zinc  by  a  protecting  tube.  Deville  and  Troost 
then  repeated  their  own  measurements  and  confirmed  their  earlier  result. 
Becquerel,  following,  insisted  that  his  measurements  with  the  platinum 
bulb  were  not  seriously  affected  by  permeability  to  hot  gases,  a  property 
with  which  he  appeared  to  be  familiar,  and  explicitly  criticized  the  use  of 
iodine  by  Deville  and  Troost. 

The  discussion  ended  here  for  the  moment,  without  a  decisive  issue,  but 
subsequent  experience  has  substantially  confirmed  Becquerel  in  his  con- 
tention and  his  numerical  results.  The  high  value  obtained  by  Deville 
and  Troost  was  undoubtedly  due  chiefly  to  the  dissociation  of  the  iodine  at 
high  temperatures,  but  this  was  not  discovered  until  afterward  in  1879  by 
Victor  Meyer.1 

The  real  catastrophe  in  the  development  of  the  gas  thermometer,  how- 
ever, does  not  lie  in  the  uncertainty  of  the  results  obtained  with  it  by  these 
distinguished  observers,  nor  yet  in  the  subsequent  discovery  that  iodine  is 
an  inappropriate  expanding  medium  with  which  to  measure  temperature; 
but  rather  in  the  discredit  in  which  the  platinum  bulb  came  to  be  held  and 
the  universal  substitution  therefor  of  porcelain — a  material  of  wholly 
uncertain  chemical  composition  and  physical  characteristics.  This  was  a 
backward  step  which  was  not  retrieved  for  more  than  thirty  years. 

Deville  and  Troost2  then  entered  upon  a  long  series  of  experiments  with 
porcelain  glazed  inside  and  outside  with  feldspar,  in  the  course  of  which  it 
appeared  that  the  expansion  of  the  bulb,  a  factor  of  great  importance  both 
then  and  now  in  gas  thermometry,  was  variable  with  the  temperatures  to 
which  it  had  previously  been  exposed.  These  irregularities  diminished 
with  continued  use  and  were  thought  to  become  negligible  in  bulbs  of 
Bayeux  porcelain  after  a  few  heatings  to  a  very  high  temperature. 

Regnaull,3 1861. — During  the  period  of  this  investigation  Regnault  was  at 
work  upon  a  displacement  method  (boiling  mercury  in  an  iron  flask  and 
estimating  the  temperature  from  the  quantity  remaining  in  the  flask  after 
cooling),  which  did  not  prove  satisfactory.  Schinz,  Berthelot,  and  Weinhold 
suggested  some  modifications  of  this  and  other  contemporary  methods, 
but  none  of  them  proved  of  permanent  value. 


'V.  and  C.  Meyer,   Ber.   Deutsch.  Chem.   Ges.,  12,   1426-1431,  1879.     V.   Meyer,  ibid.,   13,    394-399. 

IOIO-IOII,   l880. 

2Compt.  rend.  57,  897-902,  1863;  59,  162-170,  1864. 
"Ann  d.  chim.  et  d.  phys.  (3),  63,  39-56,  1861. 


HISTORICAL.  7 

Erhard  and  Schertel,1  1879. — Erhard  and  Schertel  redetermined  the 
melting  temperatures  of  the  Prinsep  alloys  in  1879,  using  a  bulb  of  Meissen 
porcelain  and  air  as  expanding  gas  with  considerable  success.  Their  work 
contributed  little  of  novelty,  but  was  carefully  done  and  the  results  have 
since  been  extensively  used. 

In  1880  Deville  and  Troost  reappeared  in  the  field,  after  a  long  silence, 
and  also  proposed  a  displacement  scheme  containing  some  improvements 
over  the  apparatus  proposed  by  Regnault.  Nitrogen  was  here  used  in 
place  of  air,  but  otherwise  the  method  possessed  insufficient  accuracy  to 
secure  for  it  general  approval.  In  the  same  year  they  published  a  summary 
of  all  their  work  on  boiling  zinc,  giving  942°  as  the  mean  of  27  determina- 
tions, which  was  (for  that  time)  in  good  agreement  with  Becquerel's  first 
value,  932°. 

Violle,2 1882. — In  the  same  year  (1882)  Violle,  using  Deville  and  Troost's 
methods  and  apparatus,  found  zinc  to  boil  at  930°  and  thus  added  a  further 
degree  of  probability  to  the  determination  of  Becquerel.  Violle  continued 
his  researches  by  determining  with  the  gas  thermometer  the  specific  heat 
of  platinum  for  a  number  of  temperatures  up  to  1200°,  and  then  extrapo- 
lating with  this  constant  for  the  measurement  of  temperatures  beyond  the 
reach  of  the  thermometer  itself.  He  thus  determined  the  melting-point 
of  gold  (1045°),  of  palladium  (1500°),  and  of  platinum  (1775°),  constants 
which  continued  in  general  use  as  standard  temperatures  for  some  years. 

In  the  decade  between  1882  and  1892  contributions  to  gas  therm ometry 
and  the  measurement  of  high  temperatures  are  few  and  unimportant,  but 
work  was  begun  in  those  years  on  both  sides  of  the  Atlantic  which,  for  the 
experimental  skill  and  persistence  with  which  the  experimental  difficulties 
and  limitations  were  pursued  and  successively  overcome,  surpasses  any 
effort  which  has  been  made  either  before  or  since  that  time.  These  were  the 
investigations  of  Barus  at  the  U.  S.  Geological  Survey  in  Washington  and 
of  Holborn  and  his  colleagues  at  the  Reichsanstalt  in  Charlottenburg. 

Barus3  (1889)  recognized,  as  no  observer  who  preceded  him  had  done,  the 
superlative  importance  of  a  uniform  temperature  distribution  about  the 
gas  thermometer  bulb  for  purposes  of  high-temperature  measurement,  and 
he  took  the  most  extraordinary  precautions  to  maintain  it.  A  temperature 
of  1000°  C.  or  more  is  not  attained  without  very  steep  temperature  gradients 
in  the  region  immediately  surrounding  the  zone  of  highest  temperature. 
It  is  therefore  a  problem  of  great  difficulty  to  introduce  a  bulb  of  from  10 
to  20  cm.  in  its  largest  dimension  into  this  hot  zone  without  leaving  some 
portion  of  it  projecting  out  into  a  region  200°  or  300°  lower  in  temperature. 
Burning  mixtures  of  gas  and  air  for  heating  purposes  also  contributed  to  the 
irregularity  and  uncertainity  of  the  temperature  distribution  about  the 
bulb.  Barus  sought,  to  avoid  this  by  a  method  of  great  ingenuity,  but  also 
of  great  technical  difficulty.  He  inclosed  his  bulb  within  a  rapidly  revolving 
muffle  which  by  its  motion  protected  every  portion  of  the  bulb  from  direct 
exposure  to  a  particularly  hot  or  a  particularly  cold  portion  of  the  adjacent 
furnace.  This  complicated  furnace  structure  and  consequently  inaccessible 


'Jahrb.  f.  Berg-u.  Huttenwesen  (i.  Sachsen),  1879.  p.  154. 
2Compt.  rend.,  94,  720-722,  1882. 

=Bull.  54,  U.  S.  Geol.  Survey,  1889.  Die  Physikalische  Behandlung  und  die  Messung  hoher  Temperaturen. 
Leipzig,  1892. 


8  HIGH   TEMPERATURE   GAS   THERMOMETRY. 

position  of  the  bulb  made  it  impossible  to  introduce  into  the  region  about 
the  bulb  the  substances  whose  temperature  constants  were  to  be  measured 
and  compelled  him  to  use  thermo-elements  which  were  first  calibrated  by 
exposure  in  the  furnace  with  the  bulb  and  then  used  independently  to 
measure  other  desired  temperatures.  The  thermo-element  has  continued 
in  general  use  in  this  intermediary  role  since  that  time. 

Earns  and  the  Thermo-element. — In  the  preparation  and  use  of  thermo- 
elements Barus  also  made  much  more  extensive  and  elaborate  studies  than 
any  one  who  has  followed  him.  He  first  investigated  a  great  number  of 
substances,  both  pure  metals  and  alloys,  and  measured  and  tabulated  their 
electro-motive  forces  for  different  absolute  temperatures.  From  these  an 
element  made  from  pure  platinum  and  an  alloy  containing  90  parts  plati- 
num and  10  parts  of  iridium  was  finally  selected  for  his  standard  work.  The 
wires  of  this  thermo-element  were  passed  through  the  axis  of  the  revolving 
muffle  and  into  a  re-entrant  tube  in  the  bulb  of  the  gas  thermometer,  where 
the  hot  junction  was  brought  to  lie  in  the  geometrical  center  of  the  spherical 
porcelain  bulb.  Here  its  electromotive  force  was  read  for  a  considerable 
number  of  measured  temperatures  and  its  curve  determined.  If  the  wires 
became  contaminated  by  exposure  to  furnace  gases  they  were  melted  and 
redrawn.  This  plan  was  pursued  most  conscientiously  and  a  considerable 
number  of  temperature  constants  from  the  melting-point  of  zinc  (424°)  to 
the  melting-point  of  gold  (1093°)  and  of  copper  (1097°)  determined. 

It  is  an  unfortunate  accident  that  history  has  failed  to  record  Barus's 
name  along  with  that  of  Le  Chatelier1  in  the  development  of  the  thermo- 
element for  purposes  of  high-temperature  measurement.  It  hardly  admits 
of  question  that  Barus  contributed  incomparably  more  to  our  knowledge 
of  the  thermo-electric  properties  of  the  different  metals  and  their  use  than 
his  distinguished  French  contemporary,  but  the  10  per  cent  iridium  alloy 
which  he  finally  selected  proved  to  be  less  serviceable  than  the  10  per  cent 
rhodium  alloy  developed  by  Le  Chatelier,  probably  by  reason  of  the  greater 
volatility  of  the  iridium  and  a  consequent  slow  change  in  its  readings.  And 
so  we  find  the  Le  Chatelier  platin-rhodium  thermo-element  in  use  to-day  the 
world  over,  while  the  magnificent  pioneer  work  of  Barus  remains  but  little 
known. 

Holborn  and  Wien,  1892. — In  the  same  year  in  which  Barus  published 
his  final  memoir  on  the  gas  thermometer  and  the  thermo-element  (1892) 
Holborn  and  Wien  published  a  paper,  "Ueber  die  Messung  hoher  Temper- 
aturen,"2  covering  nearly  the  same  ground  in  the  same  general  way,  but 
with  somewhat  different  results.  Both  used  air  as  the  expanding  gas,  both 
used  thermo-elements  to  transfer  the  standard  gas  temperatures  over  to 
the  substance  to  be  measured ;  but  Holborn  and  Wien  attained  to  higher 
temperatures  (above  1300°),  while  Barus  took  much  greater  precaution 
than  his  German  contemporaries  to  secure  a  uniform  temperature  about 
his  bulb.  The  arrangement  adopted  by  Holborn  and  Wien  possessed  the 
further  advantage  that  the  thermo-element  was  entirely  inclosed  within  the 
bulb  itself  and  so  was  well  protected  against  the  contaminating  influence 
of  furnace  gases  besides  giving  a  truer  record  of  the  actual  temperature 

'Bull.  Soc.  Chim.  47,  2,  1887.  Jotirn.  d.  phys.  6,  23,  1887. 
2Wied.  Ann.  47,  107-134,  1892. 


HISTORICAL.  9 

of  the  expanding  gas.  Over  against  this  it  should  be  stated  that  the  volume 
of  the  unheated  portions  of  their  bulb  and  manometer  connections,  which 
then  constituted  the  chief  source  of  error  in  all  gas-thermometer  measure- 
ments, was  dangerously  large. 

Barus  obtained  1093°  as  the  melting-point  of  gold,  Holborn  and  Wien 
1072°,  a  difference  of  very  troublesome  magnitude.  With  these  maybe  com- 
pared the  earlier  value  (1045°)  obtained  by  Violle  in  1882,  and  of  Callendar 
(1061°),  who  extrapolated  the  readings  of  the  platinum  resistance  ther- 
mometer from  the  sulphur  boiling-point  (444.5),  where  his  gas-thermometer 
measurements  ended. 

After  1892  Barus  turned  his  attention  to  other  things,  but  Holborn  and 
Wien  published  a  second  article1  in  1895  confirming  and  extending  their 
earlier  results.  By  employing  a  specially  refractory  porcelain  bulb  prepared 
by  Dr.  Hecht,  of  the  Konigliche  Porzellan  Manufaktur,  they  were  able  to 
continue  the  gas  measurements  nearly  to  the  melting-point  of  nickel,  which 
was  determined  by  extrapolation  to  be  1484°.  Both  Barus,  and  Holborn 
and  Wien,  continued  the  thermo-electric  measurements  up  to  the  melting- 
point  of  platinum,  the  extrapolation  yielding  1780°  (Holborn  and  Wien) 
and  1855°  (Barus)  respectively. 

Holborn  and  Day,2  1899. — With  the  advancing  demands  of  science  for 
trustworthy  high-temperature  measurements,  these  differences  in  the 
absolute  temperature  of  the  melting-point  of  gold,  which  is  an  ideal  sub- 
stance for  a  temperature  constant,  soon  came  to  be  regarded  as  unsatis- 
factory and  the  whole  problem  was  again  taken  up  at  the  Reichsanstalt  by 
Holborn  and  Day,  with  a  view  to  clearing  up  these  differences.  At  that 
time  the  gas  thermometer  was  in  serious  danger  of  falling  into  disrepute 
as  a  physical  instrument  of  precision,  and  it  came  to  be  a  common  habit  in 
meetings  of  scientific  men  to  excuse  particularly  poor  temperature  measure- 
ments by  remarking  that  they  were  made  with  the  gas  thermometer. 
Holborn  and  Day  began  by  using  bulbs  of  Royal  Berlin  porcelain,  but,  after 
the  investigation  had  proceeded  for  a  year  or  more,  abandoned  them 
definitely  and  permanently  to  return  to  the  old  platinum  bulb  of  Pouillet, 
with  an  appropriate  gas  (nitrogen)  which  could  not  penetrate  the  bulb  wall. 
A  further  improvement  of  inestimable  value  in  attaining  constant  and  repro- 
ducible conditions  was  made  when  electric  heating-coils  were  substituted 
for  gas.  With  this  change  the  contamination  of  the  thermo-elements  through 
the  action  of  combustion  gases,  the  danger  of  one  or  other  of  these  gases 
penetrating  the  bulb  wall  itself  regardless  of  the  character  or  the  pressure 
of  the  gas  within,  irregularities  of  temperature  about  the  bulb,  and  inadequate 
control  of  the  heat  supply,  were  all  eliminated  or  much  reduced  in  mag- 
nitude at  a  single  stroke.  A  preliminary  account  of  these  results  was 
published  in  1899  and  a  final  account  in  1900,  in  which  several  metal  melting 
points  were  established  as  points  of  reference  for  the  high-temperature  scale, 
which  soon  found  general  acceptance  and  are  still  almost  universally  used. 

Barus  s  Summary  *  1900. — At  the  time  of  the  meeting  of  the  international 
congress  of  physicists  held  in  connection  with  the  Paris  Exposition  in  1900 
Barus  was  invited  to  prepare  a  review  of  the  history  of  pyrometry  up  to  that 

1Wied.  Ann.  56,  360-396,  1895. 

"Wied.  Ann.  68,  817-852,  1899.   Am.  Journ.  Sci.  (4),  8,  165-193,  i899- 

3Rapports  presentes  au  Congres  International  de  Physique,  1900,  vol.  I,  148-177. 


HIGH   TEMPERATURE    GAS   THERMOMETRY. 


time,  in  which  the  following  table  appears.     It  offers  an  excellent  review 
of  the  progress  of  pyrometry  up  to  that  time. 


Investigator. 

Year. 

Silver. 

Gold. 

Copper.!    Nickel. 

Palladium 

Platinum. 

Iridium. 

1828 

0 

;  j  ; 

• 

0 

0 

Pouillet  
Ed.  Becquerel  
Violle  
Erhardt  and  Schertel 
Barus  
Holborn  and  Wien.  .  . 
Callendar 

.836 
.863 
.879 
1879 
.892 
,892 
1892 

IOOO 
960 
954 
954 
985 
97' 
(061) 

1200 
1092 
1045 
1075 
1093 
1072 
(lo6l) 

i  
1054 

1097    (1517) 
1082    (1484) 

(IJOO) 

0643)' 
(1587) 

'(1775)' 

(1855) 
(.780) 

('950) 

D  Berthelot 

1898 

062 

1064 

Q6l    5 

1064 

1084 

Values  in  parentheses  are  extrapolated. 

To  this  table  Barus's  own  determinations,  which  were  omitted  from  the 
original,  and  the  measurements  of  Holborn  and  Day,1  which  were  published 
in  the  same  year  just  after  Barus's  paper  appeared,  have  been  added.  None 
of  the  temperatures  above  the  melting-point  of  copper  was  determined  by 
direct  comparison  with  the  gas  thermometer.  Violle's  higher  temperatures 
were  obtained  by  extrapolation  by  means  of  the  specific  heat  of  platinum 
determined  for  temperatures  below  1200°,  and  Barus's  and  Holborn  and 
Wien's  by  continuing  the  curve  of  electromotive  forces  of  their  thermo- 
elements over  the  same  range. 

Since  the  beginning  of  the  present  century  but  four  attempts  have  been 
made  to  reach  1000°  C.  with  the  gas  thermometer.  These  may  be  taken  up 
in  the  order  of  their  publication  as  follows:  (i)  J.  A.  Harker  (1904),  using  a 
porcelain  bulb  and  nitrogen;  (2)  Jaquerod  and  Perrot  (1905),  using  a 
bulb  of ' '  quartz  glass ' '  and  various  gases ;  (3)  Holborn  and  Valentiner  ( 1 906) 
using  one  bulb  of  platinum  containing  20  per  cent  of  iridium  and  one  of  pure 
iridium,  both  with  nitrogen  as  the  expanding  gas;  and  finally  (4)  Day  and 
Clement  (1908)  and  Day  and  Sosman  (1910),  using  bulbs  of  platinum  con- 
taining 10  per  cent  of  iridium  and  20  per  cent  of  rhodium  respectively.  The 
last  investigation  forms  the  body  of  the  present  paper. 

Harker.2 — The  work  of  J.  A.  Harker  at  the  National  Physical  Laboratory 
(England)  does  not  differ  in  any  important  particular  from  the  work  of 
Holborn  and  Day  which  immediately  preceded  it  at  the  Reichsanstalt. 
His  instrument  was  an  exact  duplicate  of  the  Reichsanstalt  instrument  by 
the  same  maker  (except  that  the  bulb  was  of  porcelain  instead  of  platin- 
iridium),  and  has  since  been  altered  only  in  certain  minor  particulars  which 
need  not  be  recounted  here.  His  experimental  operations  were  painstakingly 
performed  and  the  results  all  in  substantial  agreement  with  those  of  Holborn 
and  Day. 

Jaquerod  and  Perrot,^  1905. — Jaquerod  and  Perrot  sought  to  establish  a 
high-temperature  scale  from  which  two  of  the  important  sources  of 
uncertainty  in  previous  work  should  be  eliminated:  (i)  the  uncertainty 


'Ann.  d.  phys.  (4),  2,  505-545,  1900.  Am.  Journ.  Sci.  (4),  10,  171-206.  1900. 

"Phil.  Trans.  (A),  203,  343-384,  1904. 

3 Arch.  d.  sci.  phys.  et  nat.  d.  Geneve  (4),  20,  pp.  28-58,  128-158,  454,  506-529,  1905. 


HISTORICAL.  1 1 

due  to  differences  in  the  expansion  of  the  various  available  gases;  (2)  any 
uncertainty  which  might  enter  the  problem  through  the  expansion  of  the 
containing  vessel  (bulb). 

To  accomplish  the  first  object  they  prepared  with  the  greatest  care  quan- 
tities of  pure  nitrogen,  oxygen,  air,  carbon  monoxide,  and  carbon  dioxide, 
and  used  these  successively  for  determinations  of  the  melting-point  of  pure 
gold.  To  accomplish  the  second  they  selected  for  the  material  of  their  bulb 
a  substance  whose  expansion  coefficient  was  less  than  one-tenth  as  great 
as  any  which  had  been  employed  for  the  purpose  up  to  that  time.  Both 
improvements  afforded  most  valuable  information.  The  five  gases  with 
appropriate  corrections  for  their  individual  pressure  coefficients  gave  the 
same  temperature  for  gold  within  very  narrow  limits  of  experimental  error 
and  the  bulb  proved  impermeable  to  all  the  gases  and  of  very  low  and  regular 
expansion  for  the  temperature  range  employed.  Its  limitation  lay  in  the 
fact  that  the  silica  bulb  can  not  be  used  for  temperatures  above  the  melting- 
point  of  gold. 

The  relative  accuracy  of  the  individual  measurements  with  this  system 
(±0.2°)  was  perhaps  higher  than  has  ever  been  attained  with  the  gas  ther- 
mometer,1 but  the  absolute  value  of  the  gold  melting-point  which  they 
obtained  (1067°)  is  considerably  higher  than  any  of  the  other  recent  measure- 
ments. Whether  this  is  due  to  some  inaccuracy  in  determining  the  constants 
of  the  (relatively  large)  unheated  connecting  space  leading  to  the  manometer, 
or  to  lack  of  uniformity  in  the  temperature  distribution  about  the  bulb, 
or  to  insufficient  data  about  the  actual  expansion  coefficient  of  the  quartz- 
glass  bulb,  or  perhaps  to  an  unfortunate  combination  of  all  of  these  sources 
of  error,  it  is  impossible  for  any  one  except  the  experimenters  themselves  to 
determine.  Certain  it  is  that  their  work  has  contributed  in  two  most  impor- 
tant particulars  to  relieve  the  technique  of  gas  thermometry  from  the 
uncertainty  which  has  hitherto  surrounded  it  and  has  thus  been  of  the 
greatest  value  to  all  who  have  followed  or  may  follow  Jaquerod  and  Perrot. 

Hoi  born  and  Valentiner,2  1906. — The  experiments  of  Holborn  and  Valen- 
tiner  contemplated  another  definite  and  important  step  in  advance.  Theirs 
was  the  first  serious  effort  to  extend  the  gas  scale  itself  from  1 150°  C.,  where 
all  previous  investigations  had  been  halted,  to  1600°  C.  The  difficulties 
confronting  such  an  undertaking  are  obvious  and  of  an  insistent  kind.  Of 
the  limited  number  of  substances  available  for  use  as  bulbs  none  is  without 
serious  limitations  at  these  extremely  high  temperatures.  Porcelain  becomes 
soft  and  its  walls  both  absorb  and  generate  gas  in  prohibitive  quantities; 
silica  glass  devitrifies;  pure  platinum  is  very  soft  and  is  permeable  to  hydro- 
gen ;  when  stiffened  with  iridium  or  rhodium  it  is  the  best  material  available ; 
but  the  iridium  is  destructive  to  the  thermo-elements,  and  the  bulb  is  likely 
to  develop  leaks  and  is  permeable  always  to  hydrogen  if  but  a  trace  of  the 
gas  or  of  water-vapor  is  about.  Furthermore,  the  difficulty  of  maintaining 
a  constant  temperature  about  a  bulb  of  200  cc.  capacity  increases  at  these 
temperatures  and  the  difficulty  of  measuring  with  thermo-elements  within 
the  furnace  is  greatly  increased  by  the  conductivity  of  all  insulating  material. 
It  is  also  a  matter  of  no  inconsiderable  difficulty  to  generate  and  to  regulate 

'Excepting  of  course  Chappuis,  who  measured  no  temperatures  above  600°.  His  magnificent  work  is 
therefore  hardly  within  the  scope  of  the  present  article. 

-Sitzungsber.  Berl.  Akad.,  1906;  811-817.     Ann.  d.  phys.  (4),  22,  1-48,  1907. 


12  HIGH   TEMPERATURE   GAS   THERMOMETRY. 

accurately  the  quantity  of  heat  required  for  a  bulb  of  this  size  under  condi- 
tions where  all  electrical  insulation  begins  to  break  down,  and  to  protect  the 
mercury  manometer  from  so  hot  a  furnace  without  removing  it  to  an 
impracticable  distance. 

All  these  reasons  and  others  of  inferior  magnitude,  but  often  of  exas- 
perating pertinacity,  contribute  to  the  glory  of  Professor  Holborn's  splendid 
attempt  to  extend  gas  thermometry  far  out  into  this  region,  which  had 
hitherto  remained  inaccessible  to  all  the  resources  of  the  laboratory  except 
the  somewhat  uncertain  extrapolation  methods.  The  bulb  with  which  the 
highest  temperature  (1680°)  was  obtained  was  of  pure  iridium  made  for 
the  purpose  by  that  indispensable  friend  of  all  recent  high-temperature 
research,  Dr.  W.  C.  Heraeus,  of  Hanau,  Germany.  It  was  small,  only 
about  50  cc.  in  capacity,  but  held  tight  through  several  determinations  of 
temperature  reaching  nearly  to  1700°.  The  temperatures  along  the  bulb 
were  much  less  constant  than  for  lower  temperatures  (differences  of  60°  on 
a  bulb  less  than  10  cm.  long),  and  many  of  the  other  difficulties  noted  above 
no  doubt  contributed  something  to  influence  the  result,  but  the  effort  dem- 
onstrated beyond  perad venture  that  the  extension  of  the  gas  scale  to  1600° 
is  practicable.  The  result  gave  melting  palladium  a  temperature  of  1575°. 

This  closes  the  account  of  the  progress  of  gas  thermometry  down  to  the 
present  undertaking,  which  was  begun  in  1904,  and  so  overlaps  only  the 
work  of  Holborn  and  Valentiner,  to  which  it  bears  a  close  relation,  as  will 
be  explained  presently. 

3.  IMPORTANCE  OF  ABSOLUTE  MEASUREMENTS  ABOVE  1150.° 

This  effort  to  reach  an  absolute  determination  of  the  temperatures  lying 
between  1200°  and  1600°  has  been  chiefly  inspired  by  two  conditions.  The 
highest  temperatures  (above  1600°)  are  conveniently  accessible  only  to  those 
methods  of  pyrometry  which  are  derived  from  the  Stefan  and  Wien- Planck 
relations,  grouped  together  for  convenience  under  the  name  "radiation  pyro- 
meters." These  methods  differ  among  themselves  somewhat  both  in  prin- 
ciple and  in  application,  but  in  general  they  are  all  characterized  by  common 
qualities.  They  do  not  require  that  the  measuring  instrument  be  in  contact 
with  the  hot  body  and  they  are  all  of  very  low  sensitiveness  at  the  lower 
temperatures.  Furthermore,  they  are  comparatively  convenient  to  use  and 
may  be  said  to  have  no  upper  temperature  limit.  These  practical  qualities 
are  obviously  of  inestimable  importance  to  all  future  pyrometric  work, 
whether  in  laboratory  or  industrial  practice.  Like  all  other  pyrometers, 
however,  they  depend  for  their  calibration  upon  the  gas  scale  and  require 
to  have  their  readings  evaluated  in  terms  of  it. 

Now  it  happens  that  these  radiation  pyrometers  do  not  become  effective 
as  temperature-measuring  instruments  below  a  bright  red  heat  correspond- 
ing to  a  temperature  of  perhaps  900°  and  consequently  they  overlap  the 
accurately  measured  portion  of  the  gas  scale  for  purposes  of  comparison 
and  calibration  only  in  the  comparatively  short  interval  lying  between  900° 
and  1150°,  above  which  the  radiation  scale  is  often  extrapolated  beyond 
3000°,  or  more  than  seven  times  the  interval  of  measured  temperatures. 
This  is  obviously  very  uncertain  procedure,  the  more  so  perhaps  because  the 


IMPORTANCE    OF   ABSOLUTE    MEASUREMENTS   ABOVE    1150  .  13 

radiation  pyrometer  is  at  its  lowest  sensibility  in  this  250°  region  in  which 
the  calibration  requires  to  be  made.  It  would  therefore  add  immensely  to 
the  stability  and  trustworthiness  of  the  radiation  methods  if  the  gas  scale 
could  be  extended  200°  or  300°  farther  into  the  domain  of  radiation  pyro- 
metry  with  proportionate  accuracy.  To  be  sure,  we  have  the  thermo- 
element which  overlaps  both  regions — the  gas  thermometer  from  very  low 
temperatures  to  1 150°  and  the  radiation  methods  from  900°  to  the  melting- 
point  of  platinum  (1755°) — but  it  also  depends  directly  upon  the  gas  scale, 
and  extrapolation  beyond  this  domain  is  fraught  with  equally  grave  uncer- 
tainty. Further  consideration  will  be  given  to  this  difficulty  on  a  later 
page  (109). 

The  second  reason  for  making  a  vigorous  effort  to  extend  the  temperature- 
measuring  standard  to  higher  temperatures  is  more  directly  dictated  by  the 
requirements  of  the  researches  into  the  conditions  of  formation  of  minerals 
and  rocks  upon  which  this  laboratory  has  now  been  at  work  for  some  years. 
It  happens  that  the  temperature  region  lying  between  1 100°  C.  and  1600°  C. 
is  the  region  in  which  the  most  important  of  the  component  minerals  which 
go  to  make  up  the  rocks  are  first  formed,  and  in  which  therefore  the  tem- 
perature requires  to  be  defined  with  great  precision  in  order  that  the  con- 
ditions of  formation  may  be  accurately  known.  In  fact,  a  quantitative 
science  of  mineral  and  rock  formation  becomes  possible  only  when  such 
formation  can  be  studied  under  known  and  accurately  reproducible  con- 
ditions, just  as  the  problems  of  quantitative  organic  or  inorganic  chemistry 
are  studied  under  known  temperature  conditions.  Until  very  recently  the 
minerals  and  the  ore  deposits  have  never  been  studied  from  this  viewpoint, 
and  our  knowledge  of  their  origin  and  relation  to  one  another  is  inferential 
and  very  fragmentary  when  compared  with  contemporary  knowledge  of 
organic  and  inorganic  chemical  compounds. 

This  investigation  was  undertaken  for  these  two  reasons,  then : 

1 i )  To  provide  a  broader  range  of  absolute  temperatures  upon  which  to 
develop  trustworthy  pyrometric  methods  for  general  use  in  the  great  and 
increasingly  important  temperature  region  lying  above  1000°. 

(2)  To  attain  much  higher  accuracy  in  the  study  of  the  conditions  of 
mineral  formation  than  has  hitherto  been  possible. 

It  is  perhaps  noteworthy  that  this  second  reason  for  undertaking  the 
present  research  is  the  same  which  inspired  the  investigation  of  Barus  in 
1892,  which  is  in  many  respects  the  most  comprehensive  ever  undertaken  in 
this  field.  It  offers  additional  reasons  (if  more  were  needed)  why  the  exist- 
ing generalizations  of  physics  and  physical  chemistry  should  be  extended 
over  a  wider  range  of  temperatures  and  pressures.  One  of  the  most  con- 
spicuous grounds  for  the  delay  in  attacking  many  of  these  obvious  and 
generally  recognized  problems  in  geophysics  lies  in  the  fact  that  the  meas- 
ured relations  established  by  the  exact  sciences  have  not  been  of  adequate 
scope  to  meet  the  needs  of  large  geologic  or  petrologic  problems.  The  great 
body  of  physical  and  physico-chemical  measurements  have  been  confined 
to  the  region  between  o°  and  100°,  while  rock  formation  may  have  extended 
over  a  temperature  region  reaching  to  1500°  C.  or  higher.  More  embarras- 
sing still  is  the  fact  that  trustworthy  data  upon  the  effect  of  considerable 
variations  of  pressure  upon  most  physical  and  physico-chemical  relations 


14  HIGH   TEMPERATURE   GAS   THERMOMETRY. 

are  altogether  lacking.  It  is  therefore  by  no  means  certain  that  the  gen- 
eralizations hitherto  regarded  as  established  in  quantitative  physics  and 
chemistry  are  directly  applicable  to  problems  of  geophysical  scope. 

4.  THE  EXPERIMENTAL  PROBLEM  IN  GAS  THERMOMETRY. 

The  gas-thermometer  problem  is  one  in  which  theory  is  often  inclined 
to  lose  patience  with  practice.  It  has  been  demonstrated  over  and  over 
again,  for  example  (Barus,  loc.  cit.,  Buckingham1),  that  the  constant-pressure 
system  of  measurement  ought  to  be  more  direct  and  free  from  error  than 
the  constant-volume  system,  notwithstanding  which  the  major  portion  of 
the  results  which  go  to  make  up  the  real  progress  of  the  past  fifty  years 
has  been  obtained  through  the  use  of  the  constant- volume  principle.  Theory 
has  also  been  very  apprehensive  from  time  to  time  of  the  ultimate  outcome 
of  attempting  to  define  temperature  in  terms  of  the  expansion  of  a  diatomic 
gas,  and  yet  nitrogen  is  the  only  gas  which  has  yet  been  found  practicable 
for  long  ranges  extending  to  the  higher  temperatures.  It  does  not  react 
with  a  platinum  bulb  and  does  not  diffuse  through  its  walls,  and  so  far  (up 
to  1600°)  no  indication  of  the  dissociation  of  nitrogen  has  been  found.  From 
the  laboratory  side  of  gas  thermometry,  the  main  difficulty  is  now,  as  it 
has  always  been,  to  find  a  practicable  bulb  which  will  hold  some  expanding 
gas  without  loss  or  change  through  a  long  range  of  temperatures  and  permit 
sufficiently  accurate  measurements  of  the  pressure-volume  relation  within. 
After  more  than  three-quarters  of  a  century  of  the  most  varied  experi- 
ences, pure  nitrogen  in  a  platin-iridium  bulb  in  which  the  pressure  at  con- 
stant volume  can  be  measured,  was  the  only  arrangement  which  had  not  yet 
encountered  some  very  serious  obstacle  to  the  extension  of  its  range  or  its 
accuracy.  It  was  therefore  adopted  without  hesitation  for  beginning  the 
study  here  described. 

If  this  somewhat  circumstantially  selected  system  does  not  at  the  moment 
appear  to  confront  any  insuperable  obstacle,  many  and  insidious  difficulties 
have  been  encountered  in  the  course  of  its  development.  One  has  only  to 
examine  the  determinations  of  the  same  temperature  made  by  different 
observers,  all  using  substantially  this  method,  to  become  convinced  that 
some  serious  work  still  requires  to  be  done  to  clear  up  the  present  uncer- 
tainty. The  melting-point  of  gold  is  given  by  Barus  (1892)  at  1093°;  by 
Holborn  and  Wien  (1895)  1072°;  Holborn  and  Day2  (1901),  1063.5°;  by 
Jaquerod  and  Perrot  (1905),  1067.2°;  by  Day  and  Clement  (preliminary, 
I9°73)>  IO59-10-  F°r  the  moment  it  is  sufficient  merely  to  call  attention  to 
these  differences  in  the  results  which  have  been  obtained,  and  to  reserve 
detailed  comment  upon  them  for  a  subsequent  part  of  the  paper.  Suffice 
it  to  say  that  both  Holborn  and  Day,  at  the  close  of  their  work  (1900) 
entertained  the  positive  opinion  that  the  discrepancies  had  occurred  in  the 
experimental  details  and  were  not  chargeable  to  an  oversight  in  any  of  the 
more  fundamental  relations  involved. 

With  this  prevailing  idea  in  mind — that  the  general  relations  are  already 
satisfactorily  worked  out  and  that  the  problem  remaining  is  therefore  pri- 

'Bull.  Bureau  of  Standards,  3,  237-293,  1907. 

!Ann.  d.  phys.  (4),  4,  99-103,  1901.  Am.  Journ.  Sci.  (4).  II,  145-148,  1901. 

3Phys.  Rev.,  24,  531-532.  '907 


THE    EXPERIMENTAL   PROBLEM    IN   GAS   THERMOMETRY.  15 

marily  an  experimental  investigation,  (i)  to  increase  the  absolute  accuracy 
of  the  measurements,  and  (2)  to  extend  their  range— Professor  Holborn  at 
the  Reichsanstalt  and  Day  and  Clement  at  the  Geophysical  Laboratory 
took  up  the  gas  thermometer  again  in  1904.  The  details  were  for  the  most 
part  independently  planned  and  the  work  has  been  independently  carried 
out.  In  a  research  which  offers  so  many  technical  difficulties,  two  inde- 
pendent plants  were  obviously  better  than  one.  In  so  far  as  a  division  of 
labor  was  attempted,  Professor  Holborn  entered  at  once  upon  the  more 
daring  undertaking,  namely,  to  increase  the  range  of  measurement.  He 
obtained  a  bulb  of  pure  iridium  in  the  hope  that  it  might  prove  possible 
to  make  continuous  gas-thermometer  measurements  as  far  as  the  melting- 
point  of  platinum.  For  this  work  the  errors  of  observation  were  allowed 
to  remain  large,  larger  in  fact  than  they  had  been  in  the  joint  work  of 
Holborn  and  Day  in  1900.  The  undertaking  was  entirely  successful  and 
yielded  very  satisfactory  measurements  up  to  about  1680°,'  the  error  for 
the  new  portion  of  the  gas  scale  (from  1150°  on)  increasing  gradually  to 
about  10°  at  1600°. 

The  work  at  the  Geophysical  Laboratory  was  for  the  moment  restricted 
to  1 200°  in  an  effort  to  eliminate  or  materially  to  diminish  the  errors  which 
have  been  inherent  in  all  gas-thermometer  measurements  up  to  this  time. 
Progress  is  necessarily  slow  in  work  of  this  character,  but  we  were  chiefly 
delayed  by  having  to  build  the  entire  equipment  ab  initio,  except  the  bulb.2 

The  instrument  which  we  constructed  for  this  work  has  now  been  in 
operation  for  more  than  five  years.  It  is  of  the  constant- volume  type,  as 
has  been  explained,  similar  in  general  plan  to  that  at  the  Reichsanstalt, 
but  differing  from  it  in  certain  important  details  with  the  especial  purpose 
of  correcting  some  of  the  known  errors  of  the  Reichsanstalt  instrument: 

(r)  A  uniform  temperature  along  the  thermometer  bulb  appeared  to  us 
imperative,  and  a  much  greater  effort  was  made  to  obtain  it. 

(2)  The  entire  furnace  was  inclosed  in  a  gas-tight  bomb  in  order  that  a 
nitrogen  atmosphere  might  be  maintained  with  equal  pressures,  both  inside 
and  outside  of  the  bulb.  This  had  the  effect  of  obviating  any  tendency  of 
the  gas  to  diffuse  into  or  out  of  the  bulb,  and  allowed  no  opportunity  for 
the  deformation  of  the  bulb  through  differences  between  the  pressure  within 
and  without.  A  further  effect  of  this  arrangement  was  to  increase  the 
sensitiveness  of  the  instrument  fully  threefold.  It  has  been  the  practice 
heretofore  in  such  temperature  measurements  to  greatly  reduce  the  initial 
pressure  of  the  gas  in  order  that  its  final  pressure  at  the  highest  temperature 
to  be  measured  may  be  substantially  equal  to  the  atmospheric  pressure 
without,  in  order  that  the  stress  on  the  bulb  through  pressure  difference 
may  be  least  when  its  power  to  withstand  such  stress  is  smallest.  In  the 
Reichsanstalt  instrument  this  restricts  the  available  range  of  pressure  for 
a  temperature  range  from  o°-ii5o°  to  about  500  mm.  of  mercury,  or  less 
than  0.3  mm.  per  degree.  By  arranging  to  increase  the  pressure  outside 

'Loc.  cit. 

'The  bulb  which  was  used  for  the  first  series  of  measurements  here  recorded  was  one  of  two  bulbs  made 
by  Dr.  Heraeus,  of  Hanau,  Germany,  for  the  Holborn  and  Day  investigation  at  the  Reichsanstalt,  one  of 
which  contained  20  per  cent  iridium  and  the  other  10  per  cent.  The  20  per  cent  iridium  bulb  is  still  at 
the  Reichsanstalt  and  was  used  in  the  investigations  of  Professor  Holborn,  to  which  reference  has  been  made. 
The  10  per  cent  iridium  bulb  was  exhibited  by  Dr.  Heraeus  at  Paris  in  1900,  after  which  it  was  loaned  to 
us  for  this  investigation.  The  form  and  capacity  of  the  two  bulbs  were  substantially  the  same,  about 
200  cc.  The  authors  take  this  opportunity  to  express  their  thanks  to  Dr.  Heraeus  for  his  most  cordial 
and  effective  cooperation  throughout  this  undertaking,  and  for  his  personal  interest  in  the  outcome  of  it. 


1 6  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

the  bulb  as  the  pressure  within  increases,  this  restriction  falls  away  and 
it  is  possible  to  extend  the  pressure  range  over  the  whole  length  of  the 
scale  which  the  manometer  carries.  The  scale  of  our  instrument  was  1.8 
meters  long.  For  a  range  of  1200°,  therefore,  we  were  able  to  work  with 
a  sensitiveness  of  a  little  more  than  i  mm.  for  each  degree  centigrade,  or 
rather  more  than  three  times  the  sensitiveness  used  in  the  Reichsanstalt 
instrument,  and  also  to  vary  the  initial  pressure  considerably  without  seri- 
ous loss  of  sensitiveness.  In  order  to  reach  1600°  this  sensitiveness  was 
subsequently  reduced  to  about  three-fourths  of  a  millimeter  per  degree, 
which  still  gives  opportunity  for  measurements  of  a  high  order  of  accuracy. 

(3)  In  the  capillary  connecting  link  between  the  bulb  and  the  manometer, 
we  were  able  to  diminish  the  volume  of  the  unheated  space  to  about  one- 
third  of  its  former  value,  and  thereby  still  further  to  reduce  one  of  the 
classical  errors  of  gas  thermometry.     This  "unheated  space,"1  it  will  be 
remembered,  serves  to  connect  the  bulb  which  contains  the  expanding  gas 
at  a  certain  temperature  and  pressure  with  the  manometer  in  which  the 
pressure  is  measured.     This  space  is  therefore  filled  with  gas  which  forms 
a  part  of  the  total  gas  content  of  the  bulb,  but  is  not  heated  with  it  and 
therefore  requires  a  correction  the  magnitude  of  which  has  sometimes  been 
so  great  as  to  create  misgivings  about  the  trustworthiness  of  the  resulting 
pressure  obtained.2    The  ratio  of  the  volume  of  the  unheated  space  to  the 

total  volume  of  the  bulb  f  7,  J  in  the  final  form  of  the  gas  thermometer  used 

by  Holborn  and  Day  (1900)  amounted  to  0.0046;  in  the  more  recent  instru- 
ment used  by  Holborn  andValentiner  it  amounted  to  0.0042  with  the  209  cc. 
platin-iridium  bulb  and  0.0181-0.0216  with  the  54  cc.  pure  iridium  bulb;  in 
the  Jaquerod  and  Perrot  apparatus  it  reached  0.0178;  while  in  our  instru- 
ment it  was  reduced  to  o.oo  1 5 .  The  entire  correction  for  the  unheated  space 
in  our  instrument  therefore  amounted  to  less  than  4°  at  1 100°  compared  with 
about  20°  in  the  older  Reichsanstalt  instrument  and  about  80°  in  the  instru- 
ment used  by  Jaquerod  and  Perrot.  An  error  of  10  per  cent  in  the  determi- 
nation of  the  average  temperature  of  the  unheated  space  in  our  instrument 
will  not  therefore  affect  the  result  more  than  0.4°  at  this  temperature. 

(4)  The  expansion  of  the  bulb  itself  was  redetermined  with  much  greater 
care  than  heretofore. 

All  these  are  details  of  the  utmost  importance  if  a  really  accurate  tem- 
perature scale  based  upon  the  expansion  of  a  gas  is  to  be  established.  The 
effect  of  a  serious  error  in  any  one  of  the  four  particulars  noted  upon  the 
temperature  measurement  is  several  times  greater  than  that  arising  from 
differences  in  the  expansion  of  the  various  available  gases  which  formed 
the  basis  of  the  elaborate  study  by  Jaquerod  and  Perrot,  to  which  reference 
has  just  been  made.  And  here,  perhaps,  lies  the  kernel  of  the  whole  matter 
so  far  as  it  concerns  the  establishment  of  accurate  fundamental  tempera- 
tures in  a  region  as  remote  as  1000°  from  the  fundamental  fixed  points. 
The  interest  of  observers  is  easily  diverted  to  questions  of  general  and 
theoretical  interest,  like  the  validity  of  the  Gay-Lussac  law  over  great 

'"Espace  nuisible,"  "Schadlicher  Raum." 

"See  in  particular  Jaquerod  and  Perrot,  Arch.  d.  sci.  phys.  et  nat.,  Geneve  (4),  20,  pp.  28,  128.  454, 
506,  1905. 


APPARATUS.  1 7 

temperature  ranges,  while  experimental  conditions  which  permit  errors  of 
considerable  magnitude  in  an  absolute  scale  have  had  altogether  inadequate 
attention.  This  is  obviously  no  aspersion  upon  the  beautiful  work  of 
Jaquerod  and  Perrot,  or  of  any  other  investigator,  but  it  may  be  the  expla- 
nation of  the  prevailing  uncertainty  in  high-temperature  measurements. 
Jaquerod  and  Perrot,  for  example,  in  measuring  the  melting-point  of  gold 
with  the  gas  thermometer,  used  five  different  gases  successively  in  the  same 
(fused  silica)  bulb,  and  came  out  with  a  maximum  variation  of  only  0.4° 
for  the  entire  series  of  observations,  and  yet  in  its  absolute  value  the  rede- 
termination  may  easily  be  5°  in  error.  In  fact,  in  one  of  their  observations 
in  which  a  porcelain  bulb  was  substituted  for  silica,  a  difference  of  4°  was 
actually  found.  The  observation  was  dropped,  but  it  serves  to  direct  atten- 
tion sharply  to  a  possible  uncertainty  of  several  degrees  arising  from  the 
corrections  for  the  distribution  of  temperature  along  the  bulb  and  the 
unheated  space,  and  for  the  expansion  coefficient  of  the  bulb  itself. 

5.  APPARATUS. 

Somewhat  more  in  detail,  the  apparatus  in  use  at  the  Geophysical 
Laboratory  may  be  described  as  follows: 

FURNACE. 

The  furnace  consists  of  a  wrought-iron  tube  of  about  25  cm.  inside  diame- 
ter, carrying  a  cast-iron  pipe  flange  at  each  end.  To  these  flanges  cast-iron 
covers  were  fitted  by  grinding  to  a  gas-tight  joint.  In  position  this  bomb 
is  vertical,  and  the  lower  cover  is  permanently  secured  in  place  with  bolts. 
The  furnace  tube  is  made  from  a  magnesite  mixture,1  is  about  36  cm. 
long  and  6  cm.  inside  diameter,  and  carries  the  furnace  coil  wround  on  its 
inside  surface.  This  scheme  of  winding  the  heating  coil  on  the  inside  of  a 
refractory  tube  is  very  successful  in  its  operation  and  is  not  difficult.  With 
a  pure  platinum  coil  (melting-point  1755°)  a  furnace  temperature  of  1600° 
can  be  reached  without  danger  to  the  coil  and  maintained  for  some  time 
if  desired.  There  is  considerable  loss  of  platinum  through  sublimation  in 
maintaining  a  resistance  furnace  at  this  temperature,  so  that  it  is  necessary 
to  use  a  wire  of  considerable  size  if  it  is  required  to  maintain  so  high  a 
temperature  for  long  periods  of  time.  The  gain  over  the  same  coil  wound 
on  the  outside  of  a  thin  porcelain  tube  is  about  200°  (1600°  instead  of  1400°) 
for  the  same  current  and  conditions  of  insulation. 

The  method  of  winding  is  simple.  A  series  of  five  wooden  wedges  is 
grouped  together  so  as  to  collapse  when  the  center  one  is  removed.  When 
grouped  and  fastened  together  the  outside  surface  is  turned  down  to  a 
cylinder  of  exactly  the  size  which  the  finished  coil  is  to  have.  This  multiple 
wedge  then  serves  as  a  collapsible  arbor  and  the  coil  is  wound  upon  it  with 
any  desired  arrangement  of  turns.  A  piece  of  paper  or  thin  cardboard 
between  the  wire  and  the  arbor  sometimes  facilitates  the  removal  of  the 
arbor  after  completing  the  furnace.  The  arbor,  with  the  coil  upon  it,  is 
then  placed  in  position  in  the  cylinder  and  the  remaining  space  between  it 
and  the  cylinder  wall  is  filled  with  magnesite  cement  of  the  same  composi- 


1  Harbison- Walker  Refractories  Co.,  Pittsburg,  Pennsylvania. 


i8 


HIGH   TEMPERATURE    GAS   THERMOMETRY. 


tion  (plus  a  little  dextrine  and  water)  as  the  tube  itself.  When  this  has 
set  the  arbor  can  be  removed,  leaving  the  coil  in  position  in  the  tube.  It 
then  remains  merely  to  go  over  the  exposed  wire  with  a  very  thin  coating 
of  the  same  cement  and  the  coil  is  ready  for  use.  Such  a  coil  is  less  liable 
to  displacement  through  expansion  and  contraction  than  when  the  winding 
is  on  the  outside  of  the  tube,  for  the  expansion  of  the  wire,  instead  of  loosen- 
ing the  coils,  merely  causes  them  to  sit  the  more  tightly  in  place.  We  have 
had  such  coils  in  constant  use  for  a  variety  of  purposes  in  the  laboratory 
for  several  years,  and  have  found  them  durable,  economical,  and  most 
convenient. 


-THMKQCUMKKl 

g's^r 
WATER 


' 


FIG.  I.  A  section  through  the  gas-thermometer  furnace  (one-sixth  natural  size). 
The  bulb  is  shown  in  position  with  the  furnace  closed  ready  for  heating.  The  capillary 
tube  connecting  with  the  manometer  passes  out  of  the  furnace  through  a  packed  joint 
at  the  upper  right-hand  corner.  The  thermo-elements  pass  through  the  center  of  the 
cover  as  indicated.  The  water-jacketing  keeps  the  furnace  sufficiently  cool  so  that  tight 
joints  are  readily  obtained  with  ordinary  rubber  packing. 

In  this  particular  furnace  the  windings  were  somewhat  closer  at  the  top 
and  bottom  of  the  coil  than  at  the  middle,  in  order  to  provide  a  more 
uniform  temperature  from  one  end  to  the  other.  This  scheme,  although 
efficient,  and  perfectly  satisfactory  for  most  purposes,  will  not  provide  a 
perfectly  uniform  distribution  of  temperature  over  long  temperature  ranges. 
An  arrangement  of  the  turns  which  is  adequate  for  low  temperatures  (500° 
to  1000°)  will  not  provide  sufficient  compensation  at  the  ends  for  much  higher 
ones  (1200°  to  1600°).  We  therefore  prepared  two  secondary  coils  of  finer 
platinum  wire  in  which  the  current  could  be  independently  varied,  and 


DAY  AND  SOSMAN 


APPARATUS.  19 

mounted  them  within  the  main  coil  at  the  two  ends  of  the  tube.  These 
coils  extended  into  the  tube  about  7  cm.  from  each  end  and  were  fastened  in 
position  by  smearing  with  magnesite  cement  as  before.  With  this  arrange- 
ment, we  were  able  to  obtain  a  temperature  distribution  along  the  bulb 
which  did  not  vary  more  than  2°  for  any  temperatures  up  to  1550°.  To 
ascertain  exactly  what  the  temperature  distribution  was  at  the  moment  of 
any  pressure  measurement,  it  was  necessary  to  use  at  least  three  thermo- 
elements simultaneously,  one  principal  element  at  the  middle  of  the  bulb 
and  secondary  elements  at  each  end.  These  elements  were  carried  out  of 
the  furnace  between  two  discs  of  rubber  packing  in  the  center  of  the  cover. 

The  bulb  was  symmetrically  located  in  the  center  of  this  furnace,  the 
capillary  stem  extending  out  at  the  top  of  the  heating  tube  and  then  with 
a  gentle  bend  of  90°  passing  out  of  the  metal  bomb  at  the  side  of  the  cover, 
as  can  be  seen  in  the  diagram  (Fig.  i).  It  was  then  connected  by  means 
of  a  second  smaller  capillary  of  platinum  with  the  top  of  the  manometer 
tube  near  the  point  of  constant  level  adjustment.  The  iron  bomb  thus 
prepared  was  water- jacketed  around  the  sides  and  at  the  top  and  bottom, 
which  effectually  prevented  any  of  the  furnace  heat  from  reaching  the 
manometer  which  stood  immediately  beside  it.  The  scale  and  mercury 
columns  of  the  manometer  therefore  suffered  exposure  to  no  temperature 
variation  other  than  that  which  existed  in  the  room,  and  any  effect  from  the 
variation  in  the  room  was  easily  avoided  by  inclosing  them  in  tubes  of  card- 
board in  which  a  rapid  circulation  of  air  was  maintained  with  a  water-jet 
pump. 

When  the  furnace  was  mounted  in  position,  the  cover,  from  which  hung 
the  thermo-elements  and  the  bulb,  was  permanently  fixed  upon  three  upright 
steel  rods  (a  a  a,  Fig.  i).  The  body  of  the  furnace  bomb  was  then  arranged 
to  be  lowered  away  from  the  cover  by  sliding  upon  two  of  the  rods  so  as 
to  expose  the  bulb  and  elements  for  ice-  and  boiling-point  determinations 
before  and  after  each  heating.  Fig.  2  shows  the  furnace  body  lowered  in 
this  way,  leaving  the  bulb  free  and  completely  accessible  for  arranging  an 
ice  bath  for  the  zero  reading. 

The  apparatus  is  shown  with  the  furnace  open  and  ready  for  the  ice- 
point  determination.  Hydraulic  power  served  to  raise  and  lower  the  fur- 
nace conveniently.  When  the  furnace  was  raised  for  heating,  a  circle  of 
bolts  provided  a  positive  pressure  upon  the  top  joint. 

MANOMETER. 

The  manometer  was  located  about  35  cm.  distant  from  the  furnace  and 
was  of  the  usual  U-tube  type,  constructed  with  a  very  heavy  cast-iron  base 
and  light  upper  parts  in  order  to  render  the  mercury  columns  as  free  as 
possible  from  the  vibrations  of  the  building.  The  fixed  point  to  which  the 
mercury  level  was  always  adjusted  occupied  the  usual  position  at  the  top 
of  the  short  arm,  the  other  arm  extending  upward  for  a  distance  of  about 
2  meters. 

The  scale,  which  was  1.8  meters  long,  was  immediately  beside  the  long 
tube,  and  was  provided  with  a  sliding  vernier  reading  to  o.oi  mm.-  It  was 
of  brass  with  a  silver-plated  band  upon  which  the  divisions  were  ruled, 
and  had  been  calibrated  by  the  German  Normal-Aichungs-Kommission  in 


20  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

Charlottenburg.  The  length  of  any  portion  of  it  was  known  in  terms  of 
the  German  standard  meter  to  the  nearest  o.oi  mm.  The  scale  was  fixed 
in  position  below  and  arranged  so  as  to  expand  upward  through  appropriate 
guides  against  a  rubber  cushion  with  the  changes  in  the  room  temperature. 
The  long  manometer  tube  also  passed  through  three  guide  screws  at  the 
top  of  the  apparatus,  which  allowed  it  to  expand  and  contract  unhindered. 
Readings  were  obtained  by  means  of  two  parallel  knife  edges  on  the  vernier 
carriage,  which  could  be  brought  to  accurate  tangency  with  the  mercury 
meniscus  by  a  slow-motion  screw  provided  for  the  purpose.  The  mechani- 
cal construction  was  extraordinarily  rigid  and  very  satisfactory. 

The  temperature  of  the  scale  and  mercury  columns  was  obtained  from 
three  thermometers,  each  set  in  a  short  tube  of  mercury  after  the  manner 
of  Holborn  and  Day.1  The  upper  tube  with  its  thermometer  could  be 
moved  up  and  down  close  beside  the  scale  and  mercury  column,  so  as  to 
give  the  temperature  of  the  top  of  the  longest  column.  The  other  two 
thermometers,  each  in  its  mercury  cup,  were  fixed  in  position  at  the  bottom 
of  the  long  column  and  the  top  of  the  short  column  respectively.  Both 
columns  were  surrounded  with  an  inclosed  air-tube  about  5  cm.  in  diameter, 
in  which  the  air  was  kept  constantly  and  rapidly  circulating  about  both 
mercury  columns.  The  observed  temperature  differences  along  the 
mercury  column  sometimes  amounted  to  0.3°.  This  does  not  seriously 
affect  the  scale  length,  but  the  average  temperature  of  the  mercury  column 
requires  to  be  known  to  about  0.2°,  with  the  high  sensitiveness  of  this 
instrument,  in  order  to  bring  the  errors  in  the  pressure  determination 
within  the  desired  limits — hence  the  three  thermometers. 

The  mercury  supply  was  contained  in  two  basins,  one  a  hollow  steel  bomb 
inclosed  within  the  cast-iron  base  of  the  instrument,  and  the  other  a  steel 
flask  mounted  upon  the  wall  of  the  room  near  the  ceiling  and  connected  with 
the  lower  reservoir  by  a  flexible  iron  tube.  Cocks  conveniently  arranged 
admitted  mercury  whenever  required.  The  fine  adjustment  of  the  mercury 
level  was  obtained  by  pressure  upon  a  nickel  diafram  which  formed  the 
bottom  of  the  lower  steel  reservoir.  This  diafram  was  about  12  cm.  in 
diameter  and  could  be  raised  slightly  by  the  upward  pressure  upon  its  center 
produced  by  turning  a  milled  hand-screw  convenient  to  the  hand  of  the 
operator.  The  lower  reservoir  was  dome-shaped  within  and  opened  into  a 
tube  and  stop-cock  at  its  highest  point,  through  which  any  air  by  chance 
imprisoned  within  the  reservoir  might  be  allowed  to  escape. 

Gas  was  admitted  to  the  bulb  by  means  of  the  three-way  cock  (A,  Fig.  4), 
leading  to  a  supply  of  pure  nitrogen,  the  pressure  of  which  could  be  varied 
at  convenience.  It  was  also  possible  to  exhaust  the  bulb  through  the  same 
cock  for  the  purpose  of  testing  for  leakage  or  rinsing  the  bulb. 

UNHEATED  SPACE. 

From  the  point  of  view  of  the  errors  of  the  instrument,  the  most  important 
part  of  the  manometer  is  the  nickel  cap  at  the  top  of  the  short  arm  which 
carries  the  fixed  point  for  defining  the  constant  volume.  This  cap  is  sealed 
into  the  glass  manometer  tube  with  ordinary  sealing-wax  of  good  quality, 
some  care  being  taken  that  the  sealing-wax  fills  all  the  cracks,  which  might 

*  Holborn  and  Day,  "  On  the  gas  thermometer  at  high  temperatures,"  Am.  Journ.  Sci.  (4),  8,  170,  1899. 


APPARATUS. 


HG  FEED 
TO  DRY  BOTTLE 


FIG.  4.  A  diagram  of  the  manometer  (about  one-eighth  size) 
showing  construction  and  essential  features  only.  Dimensions 
are  approximate. 


22  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

otherwise  retain  gas  and  become  a  part  of  the  unheated  space.  The  under 
side  of  the  cap  is  hollowed  out  slightly  to  conform  to  the  shape  of  the  rising 
mercury  meniscus,  and  in  the  center  a  somewhat  rounded  point  of  nickel 
projects  downward  about  0.3  mm.  When  the  column  of  mercury  is  raised 
in  the  arm  until  it  becomes  tangent  to  this  point,  the  constant  volume  of  the 
system  is  determined.  This  setting  is  made  through  a  fixed  magnifying 
microscope  of  some  20  diameters  power.  The  portion  of  the  "unheated 
space"  included  above  the  column  is  about  0.3  mm.  thick,  i  cm.  in  diameter, 
and  corresponds  in  form  to  the  mercury  meniscus. 

The  outlet  leading  to  the  bulb  is  a  small  opening  beside  the  contact  point 
containing  a  tiny  valve  of  nickel  about  1.5  mm.  in  diameter  and  2  mm.  long, 
with  a  ground  joint  at  the  top,  which  slides  loosely  in  such  a  way  that,  if 
an  accidental  rise  of  the  mercury  column  should  tend  to  drive  the  mercury 
over  into  the  bulb,  this  little  nickel  plug  will  be  lifted  by  the  mercury  and 
automatically  close  the  opening  at  the  ground  joint.  This  tiny  valve  opens 
into  the  capillary  (0.55  mm.  in  diameter)  leading  outward  to  the  bulb.  Fig.  4 
will  show  the  construction  more  clearly.  Where  the  space  above  the  mer- 
cury column  requires  to  be  reduced  absolutely  to  minimum  volume  some 
such  protection  is  essential.  If  mercury  once  passes  this  opening,  through 
accident  or  oversight,  it  reaches  the  bulb  almost  immediately,  and  once 
there  it  is  a  matter  of  two  weeks  boiling  with  nitric  acid  to  get  rid  of  it  again. 

Even  with  this  valve,  it  sometimes  happened  that  when  gas  was  bubbled 
through  the  mercury  in  filling,  even  at  the  bottom  of  the  tube  some  80  cm. 
distant  from  the  valve  opening,  tiny  globules  of  mercury  were  shot  upward 
with  such  speed  and  accuracy  of  aim  as  to  pass  up  beside  the  little  valve 
and  into  the  capillary  tube,  after  which  their  ultimate  destination  is  inevit- 
ably the  bulb.  The  altogether  insignificant  size  of  the  opening  and  the  dis- 
tance required  to  be  traversed  by  such  a  globule  did  not  convey  to  us  a 
suspicion  that  a  globule  might  hit  and  pass  it,  but  it  actually  happened  on 
two  different  occasions,  with  the  consequence  of  an  exasperating  delay. 

In  the  present  arrangement  of  the  gas  thermometer,  this  accident  is  also 
provided  against  by  introducing  a  gold  capillary  instead  of  platinum, 
between  the  fixed  point  and  the  furnace  bomb.  Such  microscopic  globules 
of  mercury  are  taken  up  by  the  gold  without  reaching  the  bulb  and  therefore 
remain  harmless. 

BAROMETER. 

It  was  deemed  advisable  from  the  start  not  to  attempt  to  combine  the 
barometer  with  the  manometer,  as  has  usually  been  done  by  the  French 
observers  and  latterly  in  the  Reichsanstalt  instrument  also.  It  is  a  con- 
venient method  and  is  rather  necessary  if  a  single  observer  is  to  make  all  the 
readings,  but  the  combination  brings  three  or  four  essentially  different  errors 
into  one  reading  in  a  way  that  does  not  admit  of  a  convenient  evaluation  of 
their  individual  magnitudes. 

Two  barometers  were  used  throughout  this  investigation,  both  of  Fuess 
manufacture  and  of  the  same  type  (Wild-Fuess  normal  barometer,  14  mm. 
tube).  The  corrected  readings  of  the  two  instruments  were  in  perfect  accord 
and  were  correct  in  their  absolute  value  within  0.05  mm.1 


'One  of  these  instruments  was  compared  with  the  normal  barometer  at  the  U.  S.  Weather  Bureau  at 
Washington,  the  other  was  compared  at  the  Bureau  of  Standards 


APPARATUS.  23 

THERMO-ELECTRIC  APPARATUS. 

The  thermo-electric  measurements  were  made  with  apparatus  and  by 
methods  which  have  already  been  described  in  varying  degrees  of  fulness 
in  previous  publications  from  this  laboratory.1 

Briefly,  it  may  be  noted  in  passing  that  all  the  thermo-electric  measure- 
ments without  exception  were  made  with  platinum-platinrhodium  thermo- 
elements of  Heraeus  manufacture  on  a  potentiometer  of  Wolff  standard 
construction  by  direct  comparison  with  a  saturated  cadmium  cell.  The  cell 
first  used  was  one  of  a  series  described  in  a  previous  paper,2  which  has  been 
compared  from  time  to  time  with  the  standard  cells  of  the  National  Bureau 
of  Standards  and  has  never  been  found  to  contain  an  error  greater  than  one 
or  two  parts  in  100,000.  In  the  later  portions  of  the  work  other  saturated 
cells  courteously  furnished  by  the  Bureau  of  Standards  for  purposes  of 
comparison  were  employed,  and  one  Weston  unsaturated  cell.  No  errors  or 
discrepancies  from  this  source  appeared  during  the  entire  investigation. 

The  galvanometer  was  a  Siemens  and  Halske  instrument  of  the  usual 
moving-coil  type.  Later,  a  more  sensitive  moving  coil  galvanometer  made 
by  the  Weston  Electric  Instrument  Co.  was  advantageously  substituted. 
With  the  help  of  a  small  rheostat  in  series  with  the  galvanometer,  its 
sensibility  was  maintained  at  a  constant  value  such  that  one  scale  division 
in  the  telescope  (distant  1.5  m.  from  the  galvanometer)  corresponded  exactly 
to  i  microvolt  in  the  thermo-element  reading,  which  is  roughly  equivalent 
to  0.1°.  In  these  galvanometers  the  wandering  of  the  needle  from  its  zero 
position  was  slight  and  never  amounted  to  more  than  0.2  or  0.3  of  a  scale 
division.  Both  were  almost  absolutely  dead-beat  with  periods  of  five  and 
three  seconds  respectively,  so  that  adjustments  for  a  temperature  reading 
could  be  made  with  extraordinary  rapidity  and  with  an  accuracy  out  of 
all  proportion  to  the  needs  of  the  experiment. 

The  only  error  to  which  the  thermo-electric  observations  were  subject 
was  the  contamination  arising  from  the  iridium  contained  in  the  first  bulb. 
During  the  first  year  in  which  these  observations  were  begun  the  furnace 
coil  also  contained  10  per  cent  of  iridium,  but  at  that  time  the  contaminat- 
ing effect  of  this  metal  upon  a  thermo-element  was  not  well  understood. 
Later  on,  this  coil  was  exchanged  for  a  coil  of  pure  platinum  made  especially 
for  this  purpose  by  Dr.  Heraeus,  which  was  guaranteed  to  contain  no  more 
than  0.05  per  cent  iridium  and  which  was  found  upon  analysis  to  contain 
considerably  less  than  this  quantity.  Inasmuch  as  the  furnace  coil  is  always 
the  hottest  part  of  the  system,  this  afforded  considerable  relief,  but  the 
position  of  the  elements  in  contact  with  the  bulb  made  it  impossible  to 
prevent  some  contamination  above  900°,  so  long  as  the  bulb  remained  bare. 
An  attempt  was  made  to  reduce  this  difficulty  still  further  by  the  use  of  a 
glaze  made  from  melted  mineral  albite,  which  was  appreciably  soft  at  tem- 
peratures of  1 1 00°  but  which  appeared  to  prevent  the  sublimation  of  iridium 
so  long  as  the  coating  remained  continuous.  The  viscous  material,  however, 

'Day  and  Allen.  The  isomorphism  and  thermal  properties  of  the  feldspars,  publication  of  Carnegie 
Institution  of  Washington  No.  31,  1905.  Allen  and  White.  On  woMastonite  and  pseudo-wollastonite. 
polymorphic  forms  of  calcium  metasilicate.  Amer.  Journ  Sci..  (4).  21,  89-108,  1906. 

Walter  P.  White,  Potentiometer  installation,  especially  for  high  temperature  and  thermo-electric  work, 
Phys.  Rev.,  25,  334-352.  >907-  Melting-point  determination.  Am.  Journ  Sci..  (4),  28,  453-73.  1909; 
Melting-point  methods  at  high  temperatures,  Am.  Journ.  Sci.,  (4),  28,,  474-489,  1909. 

'Day  and  Allen,  loc.  cit.,  p.  26. 


24  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

showed  a  persistent  tendency  to  gather  together  into  globules,  leaving  bare 
spots  on  the  bulb  which  were  not  wet  by  the  glaze,  so  that  this  protection 
was  not  complete.  Porcelain  insulating  tubes  open  at  the  ends  afford 
little  or  no  protection.  We  were  accordingly  driven  to  the  conclusion 
that  for  the  higher  temperatures  iridium  must  be  banished  from  the  furnace 
completely  before  consistent  observations  could  be  obtained.  This  is  the 
chief  reason  why  the  first  series  was  not  extended  beyond  1200°.  The 
observations  above  this  temperature  will  therefore  be  described  on  a  later 
page  (p.  48).  For  these  later  observations  a  bulb  containing  no  iridium  was 
substituted  for  the  one  described  here. 

Up  to  1200°  our  precautions  were  sufficient  to  prevent  serious  contami- 
nation of  the  elements  and  the  error  due  to  such  contamination  as  was 
unavoidable  has  been  eliminated  by  frequent  calibrations  of  the  three  ele- 
ments used  for  each  observation,  either  by  comparison  with  standard  ele- 
ments known  to  be  free  from  iridium  contamination,  or  by  melting-point 
determinations  of  standard  metals.  Toward  the  close  of  this  first  series, 
in  order  to  establish  absolute  proof  that  the  readings  were  not  encumbered 
with  systematic  errors,  however  small,  from  this  cause,  an  independent 
observation  was  made  in  the  following  way:  The  element  at  the  middle 
of  the  bulb  was  replaced  by  a  freshly  calibrated  new  element  known  to  be 
in  perfect  condition.  After  an  ice-point  determination  and  with  all  the 
precautions  above  described,  the  furnace  was  heated  directly  and  as  rapidly 
as  possible  to  1200°,  where  a  single  observation  was  made  and  the  furnace 
immediately  cooled  down  again.  The  new  element  was  then  removed  from 
the  furnace  and  recalibrated  in  order  to  establish  beyond  question  the  fact 
that  it  had  suffered  no  contamination  whatever  during  the  short  and  rapid 
run.  This  independent  determination,  in  which  it  was  definitely  proved 
that  iridium  contamination  played  no  part,  served  to  establish  the  absolute 
correctness  of  the  high-temperature  observations  in  so  far  as  the  error  from 
this  most  persistent  source  was  concerned. 

No  reason  has  yet  arisen  in  any  of  the  experiments  with  nitrogen  for 
suspecting  limitations  of  any  kind  due  to  the  gas.  It  has  shown  no  tendency 
to  react  with  the  platinum  bulb  or  to  pass  through  its  wall  or  to  dissociate 
at  any  temperature  to  which  it  has  yet  been  carried  in  gas  thermometry. 

THE  BULB. 

As  has  been  stated  with  some  emphasis  in  the  historical  introduction, 
the  question  of  a  suitable  bulb  to  contain  the  expanding  gas  has  been  and 
is  to-day  one  of  the  most  serious  which  gas  thermometry  confronts.  The 
first  experiments  (Prinsep)  were  made  with  a  bulb  of  gold,  which  was  soon 
abandoned  because  of  its  low  melting-point.  Following  this,  platinum  was 
employed  (Pouillet),  but  here  a  difficulty  was  encountered  which  eventually 
caused  its  abandonment  in  favor  of  porcelain  on  account  of  its  supposed 
permeability  to  gases  (Deville  and  Troost;  -Becquerel).  But  the  porcelain 
bulb  without  glaze  is  itself  porous;  with  a  glaze  it  is  a  chemically  undefined 
mineral  mixture  which  not  only  softens  below  1200°  with  more  or  less  change 
of  volume,  but  also  gives  out  gas  (either  original  or  previously  absorbed), 
so  that  the  porcelain  gas  thermometer,  as  it  is  commonly  called,  never 
returns  to  its  original  zero  after  heating  to  high  temperatures. r  The  uncer- 

'Holborn  and  Day,  Am.  Journ.  Sci.  (4),  8,  p.  185,  1899.    Wied.  Ann.  68,  p.  843,  1899. 


PLAN   OF   PROCEDURE.  25 

tainty  in  the  zero  which  arises  through  the  use  of  the  porcelain  bulb  causes 
an  error  in  a  single  observation  of  the  order  of  5°  at  1000°,  which  is  prac- 
tically impossible  of  satisfactory  correction. 

The  return  to  metal  bulbs  is  due  to  Professor  Holbornof  theReichsanstalt, 
who  has  successfully  used  a  platinum  bulb  (containing  20  per  cent  of  indium) 
of  200  cc.  capacity  with  nitrogen  as  the  expanding  gas  up  to  1600°  without 
discovering  any  irregularity  in  its  behavior.  A  similar  bulb  of  platinum 
containing  10  per  cent  of  iridium  was  successfully  used  in  this  laboratory 
for  nearly  three  years  without  developing  any  limitation  other  than  that 
due  to  the  contaminating  effect  of  the  iridium  on  the  thermo-elements. 
It  is  a  matter  of  great  difficulty  and  some  uncertainty  to  make  trustworthy 
measurements  of  temperatures  above  1000°  with  platinum  thermo-elements 
in  the  presence  of  iridium  (see  paragraph  on  thermo-electric  measurements 
preceding),  even  when  the  iridium  is  present  only  in  a  low  percentage 
(0.05%)  alloy  with  platinum.  To  obviate  this  a  bulb  of  platinum  contain- 
ing 20  per  cent  of  rhodium  and  no  iridium  was  substituted  for  the  iridium 
alloy  in  our  later  observations  (p.  48)  with  marked  success.  The  porcelain 
bulb  has  therefore  probably  disappeared  permanently  from  gas  thermometry . 

Parenthetically,  it  may  be  remarked  that  the  platinum  crucibles  and 
other  ware  as  made  up  for  laboratory  use  in  this  country  are  usually  stif- 
fened with  about  2  per  cent  of  iridium,  a  quantity  amply  sufficient  to  con- 
taminate thermo-elements  if  exposed  in  the  furnace  with  it  to  temperatures 
above  900°. 

6.  PLAN  OF  PROCEDURE. 

The  procedure  followed  in  the  first  series  of  observations  was  substan- 
tially as  follows: 

With  the  body  of  the  furnace  lowered  so  as  to  expose  the  bulb,  a  pail  of 
suitable  size  was  brought  up  about  the  latter  and  filled  with  distilled  water 
and  finely  divided  ice  in  such  a  way  as  to  inclose  the  bulb  and  so  much  of 
the  capillary  as  was  included  within  the  furnace  when  hot.  Several  read- 
ings of  the  ice-point  were  then  made  on  the  manometer,  together  with 
simultaneous  readings  of  one  or  both  barometers.  To  control  the  expan- 
sion coefficient  of  the  gas,  these  readings  were  occasionally  followed  by  a 
second  reading  at  the  temperature  of  boiling  water  in  wThich  the  ice  pail 
was  replaced  by  a  double-chambered  boiling-point  apparatus  of  standard 
type.  In  general,  however,  it  may  be  said  that  the  expansion  coefficient 
of  pure  nitrogen  has  already  been  so  carefully  determined  by  Chappuis 
and  others  that  this  observation  is  superfluous,  particularly  as  the  sensi- 
tiveness obtainable  in  a  bulb  of  a  size  suitable  for  long  ranges  of  temperature 
is  not  sufficient  to  admit  of  a  determination  comparable  with  theirs. 

After  the  ice-point  had  been  determined,  therefore,  the  general  pro- 
cedure was  to  arrange  the  thermo-elements  in  position  at  the  top,  middle, 
and  bottom  of  the  bulb  (Fig.  i),  to  close  up  the  furnace  gas-tight,  and  to 
proceed  with  the  heating.  Before  turning  on  the  current,  however,  it  was 
our  habit  during  the  earlier  experiments  to  exhaust  the  bomb  and  replace 
the  air  with  a  nitrogen  atmosphere,  the  nitrogen  being  supplied  from  a 
separate  bomb  under  high  pressure.  The  nitrogen  for  this  purpose  was 
made  in  large  quantities  in  the  laboratory  by  the  method  of  Hutton  and 


26  HIGH    TEMPERATURE    GAS   THERMOMETRY. 

Petavel,1  and  pumped  into  bombs  at  a  pressure  of  about  1,000  pounds  per 
square  inch.  One  of  these  bombs  could  be  readily  connected  with  the 
furnace  through  appropriate  portable  connections  and  a  reducing  valve 
whenever  desired.  Later,  compressed  air  was  found  to  serve  the  purpose 
equally  well.  A  pressure  gage  connecting  with  the  inside  of  the  furnace 
bomb  enabled  the  pressure  within  the  bomb,  that  is,  outside  the  bulb,  to 
be  read  at  any  time.  If  the  advance  in  pressure  outside  the  bulb  did  not 
proceed  as  rapidly  as  that  within,  additional  nitrogen  could  be  admitted 
if  required.  In  general,  it  can  be  said  of  the  operation  of  this  arrangement 
for  the  adjustment  of  pressure  within  and  without  the  bulb,  that  if  the 
furnace  is  perfectly  tight  the  two  pressures  advance  together  and  are  never 
very  far  apart.  Attention  to  this  detail  is  therefore  not  burdensome  unless 
the  bomb  is  leaking,  in  which  case  the  losses  must  be  supplied  by  the  addi- 
tion of  small  quantities  of  nitrogen  from  time  to  time.  An  effort  was  made 
to  keep  the  pressure  outside  the  bulb  within  one-half  pound  of  the  inside 
pressure  as  read  on  the  manometer. 

After  the  current  had  brought  the  temperature  to  the  point  where  it  was 
proposed  to  make  a  reading,  about  three-quarters  of  a  hour  was  required 
to  adjust  the  three  resistance  coils  so  as  to  produce  a  permanently  uniform 
temperature  along  the  bulb,  which  limited  the  number  of  temperatures 
observed  in  one  working  day  to  six  or  seven.  It  was  therefore  our  habit  to 
make  readings,  at  50°  or  100°  intervals,  so  as  to  cover  a  considerable  range 
of  temperatures  each  day.  On  following  days  intermediate  temperatures 
were  selected  in  such  a  way  that  the  whole  field  would  eventually  be  can- 
vassed in  steps  of  25°.  In  order  to  provide  a  sufficiently  rigid  control  of  the 
conditions  within  the  bulb,  however,  each  day's  readings  began  with  a  new 
determination  of  the  ice-point. 

It  is  interesting  to  note  in  passing  that  the  variation  of  the  ice-point  after 
heating,  which  was  a  conspicuous  feature  in  all  gas-thermometric  work 
previous  to  1 900,  has  now  substantially  disappeared  with  the  return  to  the 
platinum  bulb. 

When  the  temperature  had  become  constant  over  the  entire  length  of 
the  bulb,  one  observer  took  his  position  at  the  telescope  of  the  manometer 
and  the  other  at  the  galvanometer,  and  simultaneous  readings  were  made 
of  the  group  of  thermo-elements  and  of  the  pressure  within  the  bulb. 
Between  each  two  pressure  readings  a  reading  of  the  barometer  was 
made  by  the  observer  at  the  gas  thermometer,  the  barometer  having  been 
arranged  in  a  conveniently  accessible  position  for  that  purpose.  All  the 
readings  were  arranged  in  symmetrical  groups  in  such  a  way  that  the  time 
rate  of  change  of  temperature,  if  any,  would  fall  out  in  the  arithmetical 
mean  of  the  pressures  and  temperatures  at  the  beginning  and  end  of  the 
series.  At  the  beginning  and  end  of  this  set  of  observations,  readings  were 
made  of  the  three  thermometers  which  gave  the  temperature  of  the  mercury 
columns  of  the  manometer.  The  temperature  of  the  unheated  space 
requires  no  separate  determination,  as  the  average  room  temperature  was 
sufficiently  accurate  to  determine  the  correction  for  the  unheated  space. 

Following  such  a  series,  the  temperature  was  increased  by  the  desired 
interval  and  the  same  operation  gone  through  again.  Constant  attention 

"Hutton  and  Petavel,  "Preparation  and  compression  of  pure  gases  for  experimental  work,"  Journ.  Soc. 
Chem.  Ind.,  23,  87-93,  February,  1904. 


EXPANSION   COEFFICIENT   OF   PLATIN-IRIDIUM   BULB.  27 

was  of  course  required  in  the  meanwhile  to  see  that  in  increasing  the  tem- 
perature of  the  furnace,  and  therefore  of  the  bulb,  the  pressures  inside  and 
outside  the  bulb  did  not  get  too  far  apart.  The  same  was  true  of  the  cooling 
at  the  close  of  the  series. 

Before  the  bulb  was  connected  up  with  the  manometer  for  the  final  filling, 
readings  were  made  of  the  position  of  the  fixed  point  which  defines  the  con- 
stant volume  upon  the  scale.  This  was  done  by  letting  in  mercury  with  both 
tubes  open  and  reading  the  mercury  level  in  the  long  tube  when  the  meniscus 
in  the  short  tube  was  raised  so  as  to  be  just  tangent  to  the  fixed  point. 

Volume  of  the  Bulb. — The  volume  of  the  bulb,  including  the  stem,  was 
determined  by  weighing  with  water  at  the  beginning  of  these  experiments 
and  again  at  their  conclusion  with  the  following  results: 

Volume  of  platin-iridium  bulb  and  stem,  September,  1905. ...      195  .7900. 
Volume  of  platin-iridium  bulb  and  stem,  February,  1908 195.6600. 

Since  V0  enters  into  the  computation  of  temperature  only  as  a  part  of  the 
correction  factor  for  the  unheated  space,  and  as  this  total  correction  is 
never  more  than  5°,  it  is  obvious  that  the  absolute  volume  of  the  bulb  is  not, 
of  itself,  an  important  factor  in  the  problem.  On  the  other  hand,  the  cor- 
rection for  the  expansion  of  the  bulb  with  the  temperature  amounts  to  45° 
at  1 1 00°,  and  is  the  most  important  correction  factor  which  requires  to  be 
determined.  An  error  of  i  per  cent  in  the  determination  of  this  constant 
(#)  produces  an  error  of  0.5°  at  1100°. 

7.  EXPANSION  COEFFICENT  OF  THE  PLATIN-IRIDIUM  BULB. 

The  determination  of  the  expansion  coefficient  of  the  bulb  did  not  prove 
to  be  the  perfunctory  operation  which  had  been  anticipated,  but  developed 
into  an  independent  research  of  somewhat  exasperating  character,  covering 
several  months. 

APPARATUS  AND  METHOD. 

There  are  two  methods  which  might  be  pursued  to  obtain  this  constant. 
It  is  theoretically  possible  to  determine  the  actual  volume  expansion  of  the 
gas-thermometer  bulb  in  position  in  the  furnace,  but  an  effort  to  carry  it 
out  experimentally  a  few  years  ago  developed  serious  difficulties  where  the 
range  of  temperature  is  so  great  and  the  accuracy  required  so  considerable. 
We  therefore  preferred  to  obtain  a  bar  made  from  the  same  material  as  the 
bulb,  and  to  determine  its  linear  expansion  under  conditions  which  were 
under  more  perfect  control. 

In  principle,  the  method  of  procedure  is  the  one  used  at  the  Reichsanstalt. 
A  bar  of  platin-iridium  5  mm.  in  diameter  and  slightly  more  than  25  cm. 
in  length  was  prepared  for  the  purpose  and  heated  in  a  tube  furnace  in 
which  the  temperature  could  be  maintained  nearly  uniform  from  one  end 
of  the  bar  to  the  other  and  conveniently  regulated  up  to  1000°  or  more. 
The  ends  of  the  bar  were  filed  flat  for  a  distance  of  6  rnm.  and  upon  these 
flat  surfaces  millimeter  divisions  were  ruled  with  a  dividing  engine.  The 
balance  of  the  apparatus  consisted  of  a  pair  of  micrometer  telescopes 
mounted  so  as  to  observe  these  divisions  and  also  to  maintain  a  constant 


28 


HIGH   TEMPERATURE    GAS   THERMOMETRY. 


distance  between  the  fixed  cross-hairs  from  beginning  to  end  of  the  experi- 
ment. Heating  the  bar  then  served  to  move  the  ruled  lines  past  the  fixed 
cross-hairs  of  the  telescopes  and  the  amount  of  the  displacement  was  meas- 
ured for  any  desired  temperature. 

The  aggregate  expansion  of  a  25  cm.  bar  over  the  interval  from  o°  to  1000° 
is  about  2.5  mm.  The  telescope  micrometers  as  they  were  focussed  for  the 
measurements  gave  about  450  divisions  (each  about  2  mm.)  of  the  drum 
for  i  mm.  on  the  bar,  and  in  the  individual  readings  differences  of  0.2  or  0.3 
of  a  division  were  readily  distinguishable.  It  was  therefore  easily  possible 
to  make  very  accurate  measurements  of  the  expansion  of  such  a  bar  by 
direct  observation  without  the  use  of  a  contact  lever  or  any  multiplying 
device  whatsoever. 


FIG.  5.  Longitudinal  section  of  the  expansion-coefficient  furnace,  showing  bar, 
thermo-elements  (E,  E).  and  microscopes  in  position.  A  section  through  the 
arrow  is  shown  in  Fig.  6. 

The  essential  features  of  the  apparatus  can  be  partly  seen  from  the  figures 
(Figs.  5  and  6,  and  Fig.  3,  page  19),  but  require  some  description.  The 
furnace  was  erected  on  a  separate  stand  quite  independent  of  the  measuring 
apparatus.  It  consisted  of  a  narrow  tube  wound  with  a  heating  coil  and 
containing,  opposite  the  ends  of  the  bar,  two  small  openings  through  which 
the  divisions  could  be  seen.  The  inside  diameter  of  the  tube  was  15  mm. 
and  the  side  openings  were  narrow  slits  about  3  mm.  in  width  by  10  mm. 
long.  The  tube  and  its  heating  coil  extended  10  cm.  beyond  the  ends  of 
the  bar  and  the  wire  was  wound  somewhat  more  closely  at  the  ends  than 
in  the  middle  to  counteract  the  cooling  effect  of  the  end  and  side  openings. 
In  this  way  a  reasonably  uniform  distribution  of  temperature  along  the 
bar  was  secured. 

The  first  furnace  tube  was  of  porcelain  wound  with  nickel  wire  i  mm. 
in  diameter,  the  separate  turns  being  insulated  from  each  other  with  a  mag- 


EXPANSION   COEFFICIENT   OF    PLATIN-IRIDIUM   BULB.  29 

nesite  cement  which  is  sufficiently  refractory  and  conducts  but  little  at  any 
temperature  which  the  nickel  wire  can  withstand.  Thus  arranged,  the 
heating  coil  was  mounted  horizontally  in  a  much  larger  tube  (7.5  cm.  diameter) 
of  porcelain  and  the  space  between  filled  with  dry  calcined  magnesia  of 
good  insulating  quality.  The  whole  was  water- jacketed  throughout  in 
order  to  prevent  any  heat  from  the  furnace  from  entering  the  optical  system 
and  disturbing  the  fixed  distance  between  the  micrometers,  upon  which  the 
accuracy  of  the  measurement  absolutely  depends.  Both  the  insulating 
material  and  the  water-jacket  were  provided  with  small  openings  corre- 
sponding to  the  slits  in  the  furnace  tube,  so  that  the  bar  could  be  illumi- 
nated and  observed  from  without. 


FIG.  6.  A  transverse  section  of  the  expansion-coefficient  furnace  (A)  at  one  of  the  openings 
showing  method  of  illumination  by  45°  plane  glass  plate  (a) .  The  bar  and  thermo-element  appear 
in  position,  though  not  well  shown  by  this  section. 

The  measuring  portion  of  the  apparatus  was  entirely  separate  from  the 
furnace  and  consisted  of  two  telescopes,  mounted  upon  upright  brass  tubes 
firmly  secured  in  position  upon  massive  brass  carriages  which  slid  freely 
on  horizontal  steel  guide-bars,  25  mm.  in  diameter  and  ground  true.  The 
two  carriages  were  then  connected  by  an  invar  metal  bar  (Fig.  5),  to 
which  they  were  stoutly  and  permanently  clamped.  The  whole  system  was 
then  free  to  move  upon  its  guides,  but  the  relative  position  of  the  telescopes 
was  fixed.  The  object  of  this  arrangement  was  obviously  to  secure  a  con- 
stant distance  between  the  telescopes,  in  spite  of  slight  changes  in  the  tem- 
perature of  the  system  due  to  changes  in  the  temperature  of  the  room  or  to 
the  heat  from  the  observer's  body,  whatever  the  relative  expansion  of  the 
various  parts  of  the  apparatus.  After  a  good  many  observations  had  been 
made,  it  was  found  that  the  upright  brass  tubes  supporting  the  telescopes 
upon  their  carriages  were  not  uniformly  affected  by  the  heat  from  the  body 
of  the  observer.  They  did  not  therefore  expand  uniformly  and  parallel  to 
each  other,  but  tended  to  buckle  very  slightly  during  each  series  of  obser- 
vations. This  was  subsequently  corrected  by  a  second  invar  bar  above  the 
telescopes,  which  in  combination  with  the  first  formed  a  rugged  rectangular 


30  HIGH  TEMPERATURE   GAS  THERMOMETRY. 

system  which  preserved  the  cross-hair  distance  without  change  throughout 
long  series  of  observations. 

In  mounting  the  furnace  for  observation,  the  side  openings  which  gave 
access  to  the  scale  divisions  were  directed  downward  in  order  to  reduce  to  a 
minimum  the  convection  currents  of  air  which  endanger  the  constancy  of 
the  temperature  within.  The  openings  were  also  made  as  small  as  possible 
for  the  same  reason.  It  therefore  became  something  of  a  problem  to  bring 
in  light  enough  to  illuminate  the  scale  divisions  and  at  the  same  time  to  make 
observations  of  the  change  in  length  with  the  temperature.  The  device 
adopted  was  this :  In  the  optical  axes  of  the  telescopes,  and  3  to  4  cm.beyond 
the  objective,  small  total  reflecting  prisms  were  mounted  upon  the  extended 
telescope  tubes  in  such  a  way  as  to  deflect  the  line  of  sight  at  right  angles 
and  upward  into  the  furnace.  Above  these  prisms  and  between  them 
and  the  furnace  (see  Fig.  6),  windows  of  plane  optical  glass  were  set  at 
45°  in  such  a  way  that  they  served  to  reflect  the  light  from  an  incandes- 
cent lamp  upward  from  their  outer  surfaces  without  materially  interrupting 
the  line  of  observation  through  the  telescope  and  total  reflecting  prism. 
By  this  device  the  path  of  the  illuminating  light  was  the  same  as  the  path 
of  the  reflected  light  which  reached  the  observer,  which  served  to  give  plenty 
of  illumination  for  the  scale  without  increasing  the  size  of  the  openings 
beyond  what  was  required  to  see  the  actual  expansion  and  to  measure  it. 

The  illumination  was  provided  by  a  single  incandescent  lamp  of  100 
candle-power  with  a  spiral  filament  of  stock  type  giving  an  intense  and 
concentrated  illumination.  It  was  mounted  behind  the  furnace  15  cm. 
distant  from  the  openings,  and  was  so  screened  by  circulating  water  that 
its  heat  did  not  reach  the  optical  parts  of  the  apparatus  save  in  the  two 
beams  which  entered  the  furnace  for  the  illumination  of  the  bar. 

The  temperature  of  the  bar  was  determined  at  first  with  one  thermo- 
element and  afterward  with  two,  which  entered  the  furnace  tube  from 
opposite  ends  in  such  a  way  that  their  hot  junctions  could  be  bound  together 
and  moved  freely  along  the  bar  and  in  contact  with  it,  in  order  to  give  a 
double  reading  of  the  temperature  at  any  point  desired.  In  this  way  we 
obtained  the  actual  distribution  of  temperature  along  the  bar  corresponding 
to  each  determination  of  its  length. 

To  complete  the  system,  a  standard  brass  bar  was  prepared  of  the  same 
size  and  shape  as  the  platin-iridium  bar  under  investigation,  but  with  silver 
surfaces  let  in  at  the  ends  to  carry  the  divisions.  This  bar  was  compared  at 
20°  C.  with  the  standards  of  length  at  the  Bureau  of  Standards,  and  served 
to  establish  the  absolute  distance  separating  the  cross-hairs  before  and  after 
each  set  of  observations. 

The  method  of  procedure  was  now  substantially  as  follows :  The  standard 
brass  bar  was  placed  in  position  in  the  furnace  at  the  temperature  of  the 
room.  All  of  the  necessary  adjustments  to  secure  good  illumination,  to 
bring  the  cross-hairs  parallel  to  the  scale  divisions,  and  to  bring  the 
lines  into  sharp  focus,  were  then  made  once  for  all,  and  these  adjustments 
were  never  again  disturbed  until  the  series  was  completed.  The  field  of  the 
microscopes  included  5  mm.  of  the  bar,  but  only  the  three  scale  divisions 
bounding  the  2  mm.  nearest  to  the  fixed  cross-hair  were  used.  Toward 
the  close  of  the  series,  for  an  important  reason  which  will  presently  appear, 


EXPANSION    COEFFICIENT   OF    PLATIN-IRIDIUM   BULB.  31 

only  the  two  bounding  divisions  of  the  single  millimeter  which  included  the 
fixed  cross-hair  were  read  and  all  the  observations  which  had  been  made 
outside  this  limited  region  were  rejected.  Readings  were  made  from  left 
to  right  in  each  microscope  and  then  repeated  in  the  reverse  direction 
to  obviate  errors  from  the  micrometer  screw.  The  temperature  for  this 
measurement  was  determined  with  mercury  thermometers  thrust  into  the 
ends  of  the  furnace  tube  adjacent  to  the  bar  and  read  before  and  after 
the  series  of  micrometer  readings.  This  observation  served  to  establish  in 
absolute  measure  the  distance  apart  of  the  fixed  cross-hairs  of  the  micro- 
scopes. The  brass  bar  was  then  removed  and  the  platin-iridium  bar  cor- 
responding to  the  gas-thermometer  bulb  inserted  in  its  place  in  the  same 
relative  position.  It  is  necessary  here  again  to  emphasize  the  fact  that 
all  further  adjustment  must  be  made  with  the  bar  and  not  with  the  optical 
parts  of  the  apparatus. 

Having  brought  the  bar  into  exactly  the  same  position  with  respect  to 
the  telescopes  which  the  brass  bar  previously  occupied,  and  having  intro- 
duced the  thermo-elements  in  such  a  way  that  their  hot  junctions  were  free 
to  travel  along  the  bar  from  end  to  end  without  disturbing  it,  a  second  series 
of  observations  at  the  temperature  of  the  room  was  made  in  the  same  way 
as  before.  This  yielded  the  absolute  length  of  the  bar  at  room  temperature 
in  terms  of  the  standard  brass  bar.  The  furnace  was  then  ready  for  heating 
to  the  temperature  desired. 

In  the  determination  of  the  high-temperature  scale  carried  out  at  the 
Reichsanstalt  in  1900,  four  observations  of  the  expansion  of  the  bulb  mate- 
rial (250°,  500°,  750°,  and  1000°)  were  deemed  sufficient,  and  it  was  not 
thought  necessary  in  our  earlier  observations  to  increase  this  number  mate- 
rially. We  therefore  began  with  a  200°  interval.  After  the  observation  at 
the  temperature  of  the  room,  the  bar  was  accordingly  heated  to  200°  C.  and 
sufficient  time  (about  30  minutes)  allowed  for  the  temperature  to  become  con- 
stant throughout  the  furnace,  after  which  a  temperature  reading  was  made 
at  the  middle  of  the  bar  with  each  element.  Observations  of  length  were 
then  made  in  the  same  order  as  before  upon  the  pair  of  lines  adjacent  to  the 
fixed  cross-hair  in  each  of  the  microscopes,  followed  by  a  second  temper- 
ature reading  at  the  middle  of  the  bar.  After  these  observations  of  length 
and  before  any  change  was  made  in  the  temperature,  nine  consecutive  pairs 
of  observations  were  made  of  the  temperature  distribution  along  the  bar, 
first  at  the  center,  then  on  the  left  section  at  5,  10,  and  12  cm.  out  from  the 
middle,  then  the  center  repeated;  then  upon  the  right  section  with  simi- 
lar intervals,  and  again  the  center,  all  with  both  elements.  By  this  means  an 
accurate  measurement  of  the  temperature  along  the  bar  corresponding  to 
the  length  measurement  just  completed  was  obtained.  The  whole  pro- 
cedure was  then  repeated  at  temperatures  of  400,  600,  800,  and  1000°  C.,1 
after  which  the  furnace  was  allowed  to  cool  over  night  and  the  length  of  the 
bar  at  the  temperature  of  the  room  was  again  determined.  Immediately 
following  this,  an  observation  of  the  brass  bar  was  made  in  order  to  estab- 
lish the  fact  that  the  distance  separating  the  cross-hairs  had  not  been  acci- 
dentally disturbed  by  the  manipulation  of  the  furnace  during  heating. 


'Subsequently,  when  we  had  reason  to  suspect  an  irregularity  in  the  rate  of  expansion,  these  observations 
were  repeated  every  100°  and  then  every  50°  in  the  region  between  600°  and  1000°. 


32  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

At  800°  and  1000°  the  bar  is  self-luminous  to  a  sufficient  extent  to  enable 
measurements  to  be  readily  made  without  outside  light,  but  it  was  deemed 
advisable  to  use  the  outside  light  in  the  same  way  at  these  temperatures  also. 
In  passing  from  outside  to  inside  illumination,  the  lines  are  at  first  dark  on 
a  bright  ground,  and  then  bright  on  a  dark  ground,  a  change  to  which  the 
eye  accustoms  itself  only  with  considerable  difficulty.  The  measurements 
were  therefore  much  more  uniform  when  outside  light  was  used  throughout. 

The  measurements  of  the  temperature  at  once  encountered  the  difficulty 
that  the  exposure  of  the  thermo-element  in  the  presence  of  iridium  at  a 
temperature  of  1000°  contaminates  it  by  an  amount  sufficient  to  cause  a 
small  but  cumulative  error.  This  exposure  was  necessary  with  the  appa- 
ratus as  we  had  arranged  it,  and  there  was  therefore  nothing  to  do  but  to 
make  the  time  of  the  exposure  as  short  as  possible,  and  by  the  use  of  two 
elements  fastened  together  and  extending  out  of  the  furnace  at  opposite 
ends,  to  so  arrange  the  conditions  that  any  contamination,  if  sufficient  to 
affect  the  temperature,  would  become  immediately  apparent.  As  W.  P. 
White  of  this  laboratory  has  shown  in  a  recent  paper,1  the  most  critical 
portion  of  a  thermo-element  is  not  the  portion  along  which  the  temperature 
is  constant,  but  the  region  where  the  element  passes  from  one  temperature 
to  another.  In  our  furnace,  for  example,  the  region  of  exposure  to  constant 
temperature  could  give  rise  to  no  error  of  reading,  however  much  the  ele- 
ment might  be  contaminated  in  that  region,  but  if  a  contaminated  portion 
of  the  element  were  at  any  time  to  come  into  the  region  lying  between  the 
end  of  the  bar  and  the  outside  of  the  furnace,  an  immediate  difference  in 
its  reading  should  become  evident. 

It  was  therefore  arranged  that  the  junctions  of  two  elements  should  be 
bound  together  so  as  to  record  the  temperature  of  the  same  point  within 
the  furnace  and,  whenever  this  combination  of  two  elements  was  moved 
toward  one  end  of  the  bar  or  the  other,  that  a  greater  length  of  one  of  the 
elements  should  be  inside  the  furnace  than  of  the  other  and  a  different 
section  of  wire  be  exposed  in  the  transition  zone.  If  there  is  contamination, 
a  difference  in  reading  between  the  two  elements  will  then  be  immediately 
conspicuous.  In  the  earlier  observations  comprising  this  investigation,  only 
one  element  was  used,  and  by  way  of  control  at  the  close  of  a  long  series 
of  observations  a  second  element  was  introduced  in  the  manner  indicated 
above.  It  then  became  immediately  evident  that  the  first  element  had 
become  contaminated  and  that  the  observations  made  with  it  were  affected 
to  a  degree  which  could  not  be  established  after  the  observations  themselves 
were  over,  and  which  therefore  necessitated  the  rejection  of  several  entire 
series.  This  misfortune  may  serve  to  emphasize  the  necessity  of  using  more 
than  one  thermo-element  in  all  cases  where  it  is  possible  to  do  so. 

Three  other  difficulties  were  met  with  which  proved  to  be  sources  of 
considerable  inconvenience,  and  which  serve  in  greater  or  less  degree  to 
place  limits  upon  the  accuracy  attainable  in  this  particular  apparatus.  The 
first  was  the  temperature  gradient  along  the  bar,  of  which  mention  has 
already  been  made.  Earlier  observers  have  sometimes  been  content  in  sim- 
ilar cases  to  heat  a  bar  with  the  electric  furnace  and  to  make  their  measure- 
ments upon  cold  projecting  ends,  that  is,  under  conditions  such  that  the 
actual  temperature  along  the  bar  varies  from  the  temperature  of  the  room 


'Walter  P.  White,  Phys.  Rev.  26,  535-536,  1908. 


EXPANSION    COEFFICIENT   OF    PLATIN-IRIDIUM    BULB.  33 

to  a  maximum  near  the  middle  of  the  bar.  The  resulting  temperature  to 
which  a  given  measured  length  is  then  referred  is  an  integral  of  a  temper- 
ature range  which  varies  all  the  way  from  that  of  the  room  to  some  point 
considerably  higher  than  that  for  which  the  length  measurement  is  recorded. 

This  situation  seems  to  us  to  comport  badly  with  the  accuracy  otherwise 
attainable  in  measurements  of  this  kind,  if  not  to  violate  fundamental  defini- 
tions. Unless  the  expansion  coefficient  can  be  treated  as  linear,  such  a  deter- 
mination is  obviously  only  an  approximation.  Furthermore,  there  is  ample 
precedent  for  anticipating  inversions  in  an  alloy  of  this  character,  such  that 
the  expansion  coefficient  of  the  material  belowr  the  inversion  temperature 
would  differ  considerably  from  that  above  it.  An  integration,  therefore,  in 
which  the  temperature  range  is  large  may  well  overlap  two  physical  states 
in  such  a  way  that  the  length  measurement  loses  all  significance.  We  have 
not  been  able  to  establish  the  fact  that  such  an  inversion  exists  in  the  10 
per  cent  platin-iridium  alloy  within  the  temperature  range  over  which  these 
measurements  were  made,  although  there  is  a  break  in  the  continuity  of  the 
expansion,  of  small  magnitude,  which  recurs  with  some  persistence,  as  can 
be  seen  from  the  tables  which  follow  (p.  36  et  seq.). 

Supposing  such  an  inversion  to  exist,  it  would  of  course  follow  that  the 
expansion  would  be  a  discontinuous  function  of  the  temperature,  a  separate 
expansion  coefficient  would  need  to  be  determined  above  and  below  this 
point,  and  the  two  would  not  bear  any  necessary  relation  to  each  other. 
If  such  a  situation  exists  in  the  present  bar,  the  difference  is  so  small  as 
to  be  negligible  for  our  present  purpose,  but  the  plain  indication  of  an 
irregularity  led  us  to  appreciate  the  necessity  of  maintaining  the  bar  as 
nearly  constant  in  temperature  as  possible  during  the  length  measurements 
in  order  to  enable  us  to  interpret  the  measurements  intelligently. 

The  problem  of  accomplishing  this  result  gave  us  considerable  anxiety. 
As  has  been  stated  above,  the  scheme  of  making  optical  measurements 
directly  upon  the  bar  without  multiplying  device  of  any  kind  necessarily 
involves  an  opening  in  the  furnace  coil  opposite  each  end  of  the  bar,  and 
a  consequent  cooling  of  that  portion  of  the  bar  which  is  opposite  the  opening. 
The  amount  of  this  cooling,  which  is  greatest  at  the  highest  temperatures, 
reached  a  value  of  about  4  per  cent  in  the  first  furnace  coil  which  we  wound. 
The  temperature  distribution  along  the  bar  is  measurable  with  any  accuracy 
desired  by  moving  the  thermo-elements  about,  or  its  effective  average  can 
be  determined  by  direct  integration  with  a  platinum  resistance  thermometer 
of  equal  length,  stretched  parallel  to  the  bar.  We  chose  the  former  method 
on  the  ground  that  it  yielded  more  information,  and  then  sought  in  addition 
to  diminish  the  irregularity  as  much  as  possible  for  the  reason  given  above. 
Accordingly,  another  furnace  coil  was  wound  with  the  turns  closer  together 
near  the  openings.  This  changed  the  temperature  gradient  considerably 
without  materially  improving  it  (see  Furnace  II,  seq.),  after  which  a  third 
coil  was  prepared  with  still  closer  windings,  which  proved  to  be  considerably 
overcompensated  and  was  rejected. 

In  all  we  made  five  separate  trials  of  this  kind,  in  the  last  two  of  which 
(Furnaces  III  and  IV)  a  thick- walled  iron  tube  was  substituted  for  the  porce- 
lain furnace  tube  in  the  hope  of  gaining  increased  uniformity  of  temperature 
through  the  increased  heat  conductivity  of  the  tube  itself.  This  arrange- 
ment succeeded  better,  but  we  found  it  impossible  to  so  arrange  a  winding 


34  HIGH    TEMPERATURE    GAS   THERMOMETRY. 

that  the  temperature  opposite  the  openings  was  uniform  with  that  at  the 
middle  of  the  tube  for  all  temperatures  between  o  and  1000°. '  A  winding 
which  gave  good  results  at  the  lower  temperatures  gave  insufficient  com- 
pensation at  the  higher  ones.  The  obvious  possibility  of  reaching  a  uniform 
distribution,  by  subdividing  the  coil  into  sections  in  each  of  which  the 
current  could  be  independently  varied  was  not  tried  on  account  of  the 
cumbersome  manipulation  required,  and  in  part  also  because  the  results 
which  we  obtained  with  considerable  differences  in  the  gradient  appeared 
to  agree  very  well  among  themselves. 

The  temperature  carried  out  in  the  tables  in  each  case  represents  the 
integral  of  the  nine  pairs  of  readings  described  above.  The  actual  error 
which  enters  into  an  observation  from  the  variation  in  temperature  opposite 
the  openings  is  therefore  the  error  in  establishing  this  integral,  which  can 
hardly  be  greater  than  i°  C.  or  o.i  per  cent. 

It  will  probably  occur  to  other  experimenters,  as  it  did  to  us,  that  this 
difficulty  with  the  exposed  ends  of  the  bar  is  due  in  part  to  the  unavoidable 
air-currents  circulating  through  the  small  openings,  and  that  these  ought 
to  be  checked  by  the  introduction  of  windows.  We  made  two  attempts  to 
reach  the  difficulty  in  this  way,  first  using  quartz  windows  set  in  the  open- 
ing of  the  furnace  tube  itself  and  therefore  heated  with  the  tube;  and  second, 
by  the  use  of  glass  windows  set  in  the  water-jacket  and  therefore  outside 
of  the  heated  zone.  The  quartz  windows  behaved  very  well  until  high 
temperatures  were  reached,  when  they  became  displaced  by  the  unequal 
expansions  in  the  apparatus,  thereby  causing  displacements  in  the  apparent 
position  of  the  lines  of  the  scale.  When  the  windows  were  removed  to  the 
colder  parts  of  the  furnace  in  order  to  avoid  this  displacement,  sufficient 
water- vapor  condensed  upon  them  from  within  to  obscure  the  field,  so 
that  the  window  scheme  had  to  be  entirely  abandoned. 

The  second  considerable  difficulty  to  be  encountered  was  due  to  the  effect 
of  the  outside  illumination  of  the  divisions  of  the  bar  in  a  rather  highly 
magnified  field  (about  25  diameters).  Consider  the  bar  to  be  illuminated 
by  a  beam  of  light  from  a  fixed  source  which  remains  constant  in  position 
while  the  bar  expands,  and  the  light  received  through  the  telescope  into 
the  eye  to  be  reflected  from  the  polished  parts  of  the  bar  surface  between 
the  rulings.  For  reasons  which  appear  in  Fig.  7,  this  reflected  light  does 
not  show  the  lines  to  be  equally2  displaced  after  expansion.  The  reason 
for  this  is  plain  after  a  brief  consideration.  If  lines  are  ruled  with  a  sharp 
tool  upon  soft  platinum  metal  which  is  afterwards  polished  to  remove  the 
burr  left  by  the  cutting  tool,  the  effect  is  to  round  off  the  two  edges  of  each 
cut  to  a  greater  or  less  extent,  and  thereby  to  present  approximately  cylin- 
drical bounding  surfaces  to  the  incident  light.  The  apparent  boundary  of 
the  line  will  then  be  defined  by  the  reflection  of  this  light  from  the  cylindrical 
surface  into  the  telescope.  Now,  if  this  cylinder  be  moved  laterally  in  the 
direction  produced  by  the  expansion,  the  light  will  be  reflected  from  a 
different  point  on  the  cylinder  and  will  therefore  show  the  line  in  a  some- 
what different  apparent  position  from  that  which  would  be  produced  by 

'A  considerable  part  of  the  difficulty  in  correcting  the  irregular  furnace  temperature  was  due  to  the 
instability  of  nickel  wire  at  the  higher  temperatures.  The  oxidation  is  so  rapid  that  a  favorable  arrangement 
of  the  windings,  when  obtained,  does  not  give  uniform  results  for  more  than  one  or  two  series  of  observations. 
It  was  subsequently  abandoned  in  favor  of  pure  platinum. 

'The  small  expansions  of  the  millimeter  sections  themselves  have  been  taken  into  account,  although 
not  explicitly  mentioned  in  this  discussion. 


EXPANSION   COEFFICIENT   OF   PLATIN-IRIDIUM   BULB. 


35 


the  expansion  alone.    The  drawing  is  purposely  exaggerated  to  show  exactl  y 
the  character  of  this  optical  error. 

It  was  our  habit  in  beginning  these  observations  to  select  three  appro- 
priate lines  upon  each  end  of  the  bar,  and  to  make  all  the  measurements 
on  these,  whereupon  it  was  found  by  a  careful  examination  of  the  results 
that  the  displacement  of  the  three  lines  after  expansion  differed  systemati- 
cally by  a  measurable  amount  and  in  a  manner  which  could  not  be  accounted 
for  by  the  movement  of  the  bar.  This  difference  was  very  puzzling  for  a 
long  time,  but  was  finally  traced  to  the  source  described,  and  this  inference 
verified  by  actually  moving  the  bar  about  in  the  field  in  various  ways  with- 
out changing  the  temperature.  The  consequence  of  this  discovery  was  to 
compel  the  rejection  of  all  measurements  made  upon  lines  other  than  those 
immediately  adjacent  to  the  fixed  cross-hair  in  the  center  of  the  field.  The 
number  of  observations  at  each  end  was  therefore 
reduced  to  two,  but  the  agreement  of  the  results  was 
very  considerably  increased  thereby. 

The  third  difficulty  is  a  limitation  of  the  material 
itself  and  is  therefore  not  dependent  upon  the  method 
of  measurement.  It  is  the  failure  of  the  bar  to  return 
to  its  initial  length  after  heating. 

In  this  particular  bar,  25  cm.  in  length,  we  actually 
found  differences  between  the  lengths  before  and  after 
heating  of  the  order  of  magnitude  of  0.02  mm.,  which 
varied  from  one  series  of  experiments  to  another  accord- 
ing as  the  bar  happened  to  be  cooled  rapidly  or  slowly. 
This  quantity  is  some  fifty  times  larger  than  the 
smallest  magnitude  we  could  measure,  and  inasmuch 
as  it  depends  only  upon  measurements  at  the  tem- 
perature of  the  room  is  readily  accessible.  This  limi- 
tation of  platin-iridium  is  not  sufficient  to  deprive  it 
of  continued  usefulness  for  the  gas  thermometer.  It 
is  the  contaminating  action  of  the  iridium  which  dis- 
tils out  of  the  alloy  at  all  temperatures  above  900°  in  sufficient  quantities 
to  eventually  destroy  the  accuracy  of  the  thermo-element,  that  has  led 
us  to  abandon  the  iridium  alloy  for  an  alloy  of  rhodium  (see  p.  50). 

This  study  of  the  irregularities  present  or  possible  in  the  expansion  of 
the  bulb  was  pursued  much  more  persistently  than  is  usual  in  an  investi- 
gation which  is  but  incidental  to  a  much  larger  one,  on  account  of  the 
unexpected  values  obtained.  The  expansion  of  pure  platinum  as  determined 
by  Holborn  and  Day1  is  given  by  the  equation 

/=(8868/  +  I-324/2)  io"9 

while  that  of  platinum,  containing  20  per  cent  of  iridium,  made  in  the  same 
furnace  gave 

/=  (8198*  +  1.418  t2}  io"9 

We  had  expected,  as  Holborn  and  Day  assumed  in  their  calculations  in 
1900,  that  the  expansion  of  the  10  per  cent  alloy  ought  to  fall  approximately 
between  the  two.  When  it  therefore  became  apparent  that  our  observations 
were  leading  to  a  value,  for  the  10  per  cent  alloy,  which  was  of  the  same 


FIG.  -.  Showing  how 
the  lines  appeared  dis- 
placed after  expansion. 
Actual  expansion,  m  to 
point  indicated  by  the 
arrow.  Apparent  expan- 
sion, m  to  M. 


'On  the  expansion  of  certain  metals  at  high  temperatures,  Am.  Jour.  Sci.  (4),  II,  374-390,  1901.    Ann.  d  . 
phys.  (4),  4,  104-122,  1901. 


HIGH  TEMPERATURE   GAS  THERMOMETRY. 


order  of  magnitude  as  that  hitherto  found  for  pure  platinum,  we  were  for 
a  long  time  quite  unwilling  to  accept  the  result.  After  the  close  scrutiny 
of  the  apparatus  and  its  limitations  described  above,  all  of  which,  either 
singly  or  in  combination,  appeared  totally  inadequate  to  account  for  the 
unexpected  expansion  coefficient  obtained,  there  remained  the  single  possi- 
bility that  some  confusion  had  arisen  in  the  preparation  of  the  bar;  but 
Doctor  Heraeus,  who  made  the  bar,  would  not  admit  this  possibility.  Even 
then,  it  was  deemed  wise  to  make  a  chemical  analysis  of  the  bar  itself,  and 
this  was  done  by  E.  T.  Allen  of  this  laboratory,  with  the  result  that  the 
iridium  content  was  found  to  be  10.6  per  cent.  There  appears  therefore 
no  further  alternative  but  to  accept  the  irregular  variation  of  the  expansion 
with  the  percentage  composition  as  characteristic  of  platin-iridium,  follow- 
ing the  well-known  example  of  the  iron-nickel  alloys. 

EXPERIMENTAL  DATA. 

TABLE  I. — OBSERVATIONS  ON  THE  EXPANSION  COEFFICIENT  OF  THE  ALLOY  90  PT.  IO!R. 
In  Furnace  I :  Temperature  distribution  along  the  bar. 


Left. 

Middle. 

Right. 

12  cm.     i    10  cm. 

5  cm. 

(Corrected 
temperature.)          5  cm. 

10  cm.         12  cm. 

+  10°            +11° 

+  12     :    +  13 
+  10         +23 
+28          +33 

+  7° 
+  7 
+  13 
+  '7 

287.7°            ~  4° 

511  .2                  —    2 

700.                -   5 

!  044  .  !                  —  1  I 

-13°        -«5° 
-'5          -24 

—  21             —30 
-31             -46 

,  _  /      total  expansion 
L       initial  length 


In  Furnace  I:  Expansion. 

Equation  used  for  the  "calculated" 
expansions,  /l  =  (8869. 5^+1 .3i92/2)icr0 


Date. 

Corrected 
temperature. 

i 

Obs.—  Cal 

Observed. 

Calculated. 

December  30,  1907  

'287.7° 

.  00263  5 

.002661 

-26 

511.2 

.004871 

.004879 

-   8 

712.9 

.  00705  I 

.006994 

+  57 

December  31,  1907  

7OO.O 

.006878 

.006855 

+23 

866.6 

.008653 

.008677 

-24 

January  2,  1908  

504.0 

.  0048  1  2 

.  004805 

+  7 

504.4 

.  0048  i  3 

.004810 

+  3 

690.0 

.006763 

.006748 

+  15 

689.4 

.006755 

.006742 

+  '3 

856.5 

.008600 

.008565 

+35 

856.4 

.008610 

.008564 

+46 

1044.1 

..010616 

.010699 

-83 

1043.8 

.010635 

.010695 

-60 

llnasmuch  as  the  expansion-coefficient  which  is  here  being  determined  itself  enters  into  the  determination 
of  the  temperature,  the  two  quantities  are  not  independently  variable.  The  temperatures  given  above  are 
therefore  based  upon  tentatively  assumed  constants  which  have  been  chosen  about  where  the  final  values 
were  expected  to  come.  The  assumed  data  are  these:  melting-point  of  zinc,  419°;  of  silver,  960°;  of 
copper,  1083°.  With  actual  temperatures  i°  higher  or  lower,  the  expansion  coefficient  would  not  be 
affected  by  an  amount  equal  to  one-tenth  of  one  per  cent  in  any  part  of  the  curve.  The  assumed  values 
are,  therefore,  amply  exact  for  the  purpose. 


EXPANSION   COEFFICIENT   OF   PLATIN-IRIDIUM   BULB. 

TABLE  I — Continued. 
In  Furnace  II:  Temperature  distribution  along  the  bar. 


37 


Left. 

Middle. 

Right. 

12  cm. 

10  cm. 

5  cm. 

(Corrected 
temperature.) 

5  cm. 

10  cm. 

12  cm. 

-'3° 

—    1° 

0° 

294.0° 

-4° 

—  12° 

-27° 

-22 

-  6 

—    2 

392.0 

-6 

—  2O 

-4' 

-28 

-  9 

—  4 

491.0 

+  2 

-'7 

-33 

-30 

-'3 

-   5 

592-5 

O 

—  IO 

-30 

-34 

-'5 

-   5 

695.0 

+4 

-  3 

-27 

-35 

->7 

-   7 

795.0 

+8 

+  4 

-'7 

-52 

—  21 

-  9 

894.0 

+9 

+  6 

—  12 

-?i 

—  21 

—  10 

994.0 

+8 

+  8 

-14 

In  Furnace  II:  Expansion. 
Equation  used  for  the  "calculated"  expansions,  >l  =  (8778.6/4-1. 28oi/2)icr9 


/ 

Date                                Corrected 
temperature. 





Obs—  Cal. 

Observed. 

Calculated. 

1 

February  25,  1908  294.0° 

.002679 

.002692 

-'3 

392.0 

.003665 

.003638 

+27 

491.0 

.004660 

.004619 

+4' 

592.5 

.005632 

.005651 

-19 

695.0 

.006657 

.006719 

-62 

795.0 

.007741 

.007788 

-47 

894.0 

.008848 

.008871 

-23 

994.0 

.  O  I  OO86 

.009991 

+95 

In  Furnace  III:  Temperature  distribution  along  the  bar. 


Left. 

Middle. 

Right. 

(Corrected 

12  cm. 

10  cm. 

5  cm. 

temperature.) 

5  cm.      | 

10  cm. 

12  cm. 

-    2° 

_       ,0 

0° 

297.9° 

o    i 

5 

-  7° 

-  6 

-   3 

-1 

397-3 

-    5 

-  8 

-  9 

-  6 

-2 

496.3 

— 

-   5 

-  9 

-13 

—  10 

-3 

594-3 

+ 

-   3 

-16 

—  12 

-4 

646.9 

+ 

—    2 

-  6 

-16 

—  12 

-4 

646.6 

+ 

—    2 

-   5 

-'7 

-'3 

-4 

697.0 

+ 

O 

-  4 

-19 

-'4 

-5 

747-8 

+ 

+    2 

—    2 

-23 

—  '7 

-6 

796.3 

+   5 

+  3 

0 

-11 

—  20 

—  20 

-7 
-8 

846.2 
897.2 

+  6 
+  6 

+   5 
+  8 

+    I 

+  4 

-29 

-23 

-9 

946.6 

+  7 

+  H 

+  8 

—  31 

-25 

-9 

1001  .  5 

+  11 

+  '7 

+  '4 

1 

72745 


HIGH   TEMPERATURE    GAS   THERMOMETRY. 


TABLE  I — Continued. 

In  Furnace  III:  Expansion. 

Equation  used  for  the  "calculated"  expansions,  /  =  (8874.4/+i  .2889<z)io-" 


Corrected 

% 

temperature. 

Observed. 

Calculated. 

April  6    1908 

297   Q° 

002770 

002759 

+  11 

April  8    1908 

397-3 

496.3 

594-3 
646.9 
646.6 

697  o 

.003739 
.  004720 
.005714 
.006267 
.  006262 

006800 

.003730 
.004723 
.005732 
.006283 
.  006280 

006815 

+  9 
-  3 

-18 
-16 
-18 

—  1  5 

74% 

897:2 
946.6 
1001.5 

.007346 
.007897 
.008445 
.009013 
.009579 
.  o  i  0206 

.007360 
.007888 
.008437 
.009005 
.009561 
.010187 

-14 
+  9 
+  8 
+  8 
+  18 
+  '9 

In  Furnace  IV:  Temperature  distribution  along  the  bar. 


Left 

Middle. 

>g 

12  cm.          10  cm. 

Scm. 

temperature.) 

5  cm. 

10  cm.         12  cm. 

-   3°         -2° 

0° 

299..° 

-    1° 

1 
-  4°        -   5° 

-5           ~   3 

O 

399  2 

—    I 

-  4         -  6 

-  9          -5 

—     I 

497.0 

—    1 

-   5          -   7 

-14          -  9 

—    2 

598.3 

0 

-  3          -  6 

-16          -  9 

—    :> 

648.0 

+  1 

-1-5 

—  19              —  12 

—  4 

709.5 

+  3 

o         —  4 

-21              -15 

-  4 

748.7 

+  4 

+  2-3 

-25          -18 

—  30             —22 

-  6 

-  7 

799-1 
846.1 

+  6 

+  7 

+  6        +   i 

—  30       i      —22 

-  8 

900.4 

+  8 

+  9         +4 

-36       '      -26 

-10 

949-6 

+  10 

+  '3         +  7 

-36             -27 

—  1  1 

1000.5 

+  12 

+  17         +'i 

i 

In  Furnace  IV:  Expansion. 
Equation  used  for  the  "calculated"  expansions,  /  =  (8814. 


.3260^  )io-' 


Date. 

Corrected 
temperature. 

' 
Obs.—  Cal. 

Observed. 

Calculated. 

. 

April  17,  1908  

299.1° 

.  002763 

.002755 

+  8 

399.2            .003750 

.003730 

+  20 

497-0 

-.004697 

.  004708 

—  I  I 

598.3 

.005702 

.005748 

-46 

648.0         .006265 

.006268 

709.5          .006889 
748.7      !  .007344 

.  00692  1 
.007343 

-32 

+    I 

799.1       |  .007897 

.  007890 

+  7 

846.1 

.008423 

.008407 

+  16 

900.4 

.009018 

.009011 

+  7 

949.6         .009585 

.009566 

+  19 

1000.5         .010160 

.010146 

+  14 

L. 

EXPANSION   COEFFICIENT   OF   PLATIN-IRIDIUM   BULB.  39 

The  mean  of  the  equations  derived  from  the  observations  in  the  four 
furnaces,  each  weighted  according  to  the  number  of  observations  in  that 
particular  series,  is 


io~9 

which  is  the  equation  used  to  compute  all  the  gas-thermometer  observations 
made  with  the  platin-iridium  bulb. 

This  interpolation  formula  is  a  simple  equation  of  two  coefficients  obtained 
by  the  method  of  least  squares,  giving  equal  weight  to  all  the  observations. 

Inasmuch  as  no  one  of  the  differences  between  observed  and  calculated 
values  reaches  i  per  cent  in  value,  this  form  of  equation,  which  has  been 
frequently  employed  for  the  purpose,  is  perhaps  as  well  adapted  to  represent 
the  experimental  data  as  another.  When  it  was  discovered  that  the  bar 
after  heating  did  not  return  to  its  initial  length,  but  varied  within  consid- 
erable limits  from  one  heating  to  another,  it  became  apparent  that  if  the 
contraction  upon  cooling  was  not  uniform,  the  expansion  on  reheating  was 
probably  also  irregular  to  the  same  degree,  and  that  the  room  temperature 
observations  could  not  be  expected  to  follow  this  or  any  other  simple  equa- 
tion very  consistently.  That  such  irregularities  exist  and  attain  such  mag- 
nitude as  seriously  to  limit  the  power  of  any  simple  formula  to  reproduce 
the  expansions  over  the  whole  range  will  be  immediately  apparent  from  an 
examination  of  the  columns  of  differences  (Obs.  —  Cal.).  It  is  more  directly 
observable  in  the  experimentally  determined  values  of  the  expansion 
between  o  and  300°  taken  from  the  four  series  which  have  just  been  given. 

MEASURED  EXPANSION  IN  MILLIMETERS  BETWEEN  o°  AND  300°. 

December  30,  1  907  .....................................  o  .  687 

February  25,  1908  ......................................  0.681 

April  6,  1908  ..........................................  0.700 

April  17,1  908  .........................................  o  .  696 

By  way  of  experiment  we  tried  an  equation  of  three  coefficients  on  the 
last  two  series,  both  of  which  contain  observations  at  50°  intervals,  omitting 
in  each  case  the  room  temperature  observation  in  which  the  irregularity  in 
the  expansion  itself  chiefly  appears,  and  found  it  possible  to  reproduce  the 
measured  behavior  of  the  bar  in  the  region  from  300°  to  1000°  with  differ- 
ences less  than  one-fifth  as  large  as  those  recorded  in  the  tables  above. 
There  is,  therefore,  abundant  evidence  that  the  uncertain  region  is  confined 
to  the  lower  temperatures  and  that  the  higher  temperatures  have  so  far 
offered  no  serious  difficulty  or  irregularity,  either  in  measurement  or  con- 
venient representation.  The  expansion  measurements  over  the  entire  range 
from  o°  to  1000°  may  be  in  error  by  about  0.5  per  cent,  most  of  which  is 
directly  attributable  to  these  irregularities  in  the  behavior  of  the  metal  at 
the  lower  temperatures.  In  the  gas  thermometer  this  corresponds  to  about 
0.25°  at  1000°. 


4o 


HIGH  TEMPERATURE   GAS  THERMOMETRY. 


8.  THE  PRESSURE  COEFFICIENT  OF  NITROGEN. 
A  number  of  determinations  of  the  pressure  coefficient  of  nitrogen,  under 
different  initial  pressures,  were  made  by  observing  the  pressure  inside  the 
bulb  when  it  was  immersed  alternately  in  ice  and  in  steam,  with  the  fol- 
lowing results.     Values  of  a  obtained  by  Chappuis  are  also  given. 


Day  and  Clement. 

Chappuis.  * 

No.  of 

observations. 

Initial 
pressure. 

a 

Initial 
pressure. 

a 

4 

I 

12 

3  14  mm. 
550 
744 
985 

0.003665 
.003668 
.003670 
.003673 

528  .  8  mm. 
534-3 
793-5 
995  9 

0.0036681  i 
.00366846 
.00367180 
.00367466 

9.  GAS-THERMOMETER  MEASUREMENTS.    FIRST  SERIES. 
COMPUTATION  OF  RESULTS. 

The  following  formula  for  the  constant-  volume  gas  thermometer  was  used 
for  the  computation  of  results  obtained  with  the  platin-iridium  bulb  : 


I  +  a/         I  +  a/,         i  +  at2 
In  this  equation: 

F0  =  volume  of  bulb  at  o° 


_     ,   y 

o 


,     _PoVi_ 

r  I  +  at', 


PM 

I  +  at', 


195  .  547  cc. 


=  volume  of  bulb  at  t.° 

=  initial  pressure,  i.  e.,  pressure  when  bulb  is  at  o°. 

=  pressure  at  temperature  of  /°. 

=  portion  of  "unheated  space"  inclosed  in  furnace  (in 
which  temperature  varies  from  the  temperature  of  the 
bulb  to  that  of  the  room)  ...........................  o  .  161  cc. 

=  portion  of  "unheated  space"  outside  of  furnace  .......   o.  128  cc. 

=  estimated  mean  temperature  of  »,  when  bulb  is  at  /°. 

=  estimated  mean  temperature  of  »,  when  bulb  is  at  o°. 

=  temperature  of  v2  when  bulb  is  at  f. 

=  temperature  of  v2  when  bulb  is  at  o°. 

=  expansion  coefficient  of  nitrogen  under  constant  volume. 

=  linear  coefficient  of  expansion  of  platinum-iridium  alloy. 


Writing 


A 


i  +  at\    '    i+a*'2  i+a/x    '    I  +  a/2 

the  equation  may  be  transformed  into  a  more  convenient  form  for  com- 
putation : 


3/3/  represents  the  correction  for  the  expansion  of  the  bulb  and^l—  -B 

Po 

is  the  correction  for  the  unheated  space.     In  computing  p  the  mercury 
columns  were  corrected  in  the  usual  manner  for  temperature  and  latitude. 

'Travaux  et  M^moires  du  Bureau  International  des  Poids  et  Mesures.  vols.  6  and  12,  1888  and  1902. 


GAS-THERMOMETER   MEASUREMENTS.      FIRST   SERIES. 


EXPERIMENTAL  DATA. 

Table  II  contains  some  of  the  earlier  results,  which  were  obtained  after 
the  temperature  gradient  along  the  bulb  had  been  only  partially  corrected. 
During  this  series  of  observations,  the  temperature  difference  between  the 
middle  and  either  end  of  the  bulb  varied  between  50  and  150  microvolts 
(5°  to  15°).  As  it  was  impossible  entirely  to  eliminate  the  gradient  with  the 
arrangement  of  coils  in  use  at  this  time,  the  heating  currents  were  adjusted 
so  as  to  have  the  gradients  toward  the  top  and  bottom  of  the  bulb  of 
opposite  sign  and  of  nearly  equal  value,  thereby  materially  reducing  the 
magnitude  of  the  correction  to  be  applied. 

TABLE  II. 

Initial  pressure,  302.3  mm.     €1  =  0.003665. 

Average  temperature  difference  between  middle  and  either  end  of  bulb,  10°. 

Equation  used  for  "calculated"  temperatures, 

/  =  50. 19+0. 11176  e  —  i  .  289X10- Ge-. 


1906.              pn 

Thermo- 
couple P. 

Temperature 
observed. 

Temperature 
calculated. 

/(obs.)—  t  (cal.) 

mm. 

mv. 

April  30..      302.09 

3231.' 

396-8° 

397-8° 

—  I  .O° 

4738. 

550.4 

550.8 

—    -4 

6232. 

696.5 

696.6 

-    .1 

7302. 

797.6 

797-5 

+    -I 

1 

i 

8428. 

900.2 

900.5 

-    -3 

9547- 

998.4 

999-6 

—  I  .2 

i 

11004. 

1123.5 

1123.9 

-    -4 

May  i  ...      302  .  20 

May  2  

296,.' 

368.6 

369.8 

—  I  .2 

4944- 

571.2 

571.2 

.O 

6448. 

717.8 

717.2 

+   .6 

7525- 

818.6 

818.2 

+   -4 

' 

8094. 

869.7 

870.3 

-   .6 

9040. 

954-5 

955-1 

-   .6 

9750. 

1016.4 

1017.3 

-   -9 

10583.' 

1089.9 

1188.5 

+  1-4 

May  3  ...     302  .  29 

May  4  

3928. 

470.0 

469.3 

+    -7 

5186. 

595-2 

595-1 

+   -2 

5944- 

669.1 

668.9 

+     -2 

6725. 

743-8 

743-4 

+    -4 

7755- 

839.7 

839.3 

+    -4 

9255 

973  -i 

974-1 

—  1  .0 

10135. 

1049.9 

1050.4 

-    -5 

May  5  ...     302  .  30 

May  7  

3918. 

468.0 

468.3 

0 

6JU. 

762.8 

762.4 

+  '4 

8910. 

943  4 

943.6 

—     .2 

10631  • 

1092.8 

1092.6 

+     -2 

11264. 

1147.0 

1145.4 

+  1.6 

May  9.  .  .     302.  52 

Before  beginning  this  series  of  observations  and  again  after  its  completion, 
the  thermo-couple  P  was  calibrated  by  determining  its  electromotive  force 
at  the  zinc  and  copper  melting-points.  From  the  results  which  follow  it  will 


•Observations  below  400°  were  not  used  in  computing  the  parabola. 
"Temperature  fell  2°  during  observation. 


42  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

be  seen  that  the  electromotive  force  of  the  thermo-element,  at  the  temper- 
ature of  melting  copper,  has  been  lowered  15  microvolts  (1.2°)  through 
iridium  contamination  during  the  series  of  measurements  : 

Zinc.  Copper. 

April  28,  1906  ................  F.1     3398  10488 

M       3398  10483 

-  3398          -  10486 
May  14,  1906  ................  F   3396  10469 

M   3398  10472 

-  3397          -  10471 

After  these  observations  (Table  II)  the  furnace  was  rebuilt.  In  place  of 
the  heating  coil  of  platinum-iridium  alloy,  a  coil  of  pure  platinum  was  sub- 
stituted. At  the  same  time  the  arrangement  of  the  two  auxiliary  heating 
coils  was  so  modified  that  by  proper  adjustment  of  rheostats,  the  gradient 
along  the  bulb  could  be  reduced  to  0.5°  or  less. 

Table  III  contains  a  complete  series  of  76  observations,  without  omission, 
covering  a  period  of  more  than  three  months  in  time,  in  the  order  in  which 
they  were  made  and  with  the  control  melting-points  through  which  the 
constancy  of  the  thermo-elements  was  assured. 

In  order  to  eliminate,  as  far  as  possible,  any  error  due  to  the  contami- 
nation of  the  thermo-couples  with  iridium,  the  couples  were  calibrated  from 
time  to  time  by  metal  melting-point  determinations.  Columns  7  to  10  of 
Table  III  contain  the  E.  M.  F's  of  the  standard  thermo-couple  W  for 
these  calibrations. 

As  a  check  against  accidental  errors  of  observation,  all  observations  were 
made  in  pairs,  with  an  interval  of  from  5  to  10  minutes  between. 

In  computing  the  constants  of  the  equation 


which  was  used  for  the  "calculated"  electromotive  forces,  the  observations 
marked  *  in  Table  III  were  omitted  in  order  to  equalize  the  intervals  between 
points.  By  comparing  the  values  /  (obs.)  —  /  (calc.)  (Table  III)  of  the 
various  pairs  of  observations,  it  will  be  seen  that  any  two  determinations  at 
the  same  temperature  agree  within  o.  i°.  The  differences  between  observed 
and  calculated  temperatures  are  all  considerably  less  than  i°.  The  greatest 
difference  is  0.6°,  and  the  average  difference  0.2°. 

If  we  now  regroup  these  observations  (Table  IV)  in  the  order  of  increas- 
ing temperatures  and  combine  the  pairs  referred  to  above,  the  relation 
between  the  observed  and  calculated  curves  appears  in  a  most  favorable 
light.  The  average  difference  in  column  6,  Table  IV,  is  even  smaller  than 
in  Table  III  and  wholly  free  from  systematic  variation.2 


1F  =  Freezing-point,  M  =  Melting-point. 

'When  these  observations  were  first  published  (Am.  Jour.  Sci.  (4),  26,  405-463,  1908),  the  "calculated" 
values  were  obtained  from  the  equation 

t  =  5 1 .72+0. 112499  e  —  i.  355 12X10-°  e- 

and  showed  systematic  variations  from  the  observed  temperatures  amounting  in  maximum  to  0.7°.  This 
was  interpreted  to  mean  that  the  simple  parabola  was  no  longer  adequate  to  represent  the  observations 
with  sufficient  accuracy.  In  a  private  letter  Dr.  George  F.  Becker  subsequently  called  the  attention  of  the 
authors  to  the  fact  that  if  the  least-square  solution  were  computed  with  e  expressed  in  terms  of  /  these 
systematic  differences  not  only  disappeared  but  a  much  better  agreement  (obs. -  calc.)  was  obtained. 
Tables  III,  IV,  and  V  have  accordingly  been  recomputed  with  Dr.  Becker's  equation,  with  the  result 
that  the  grounds  for  apprehension  expressed  in  the  previous  paper  have  happily  disappeared. 


1AS-THERMOMETER    MEASUREMENTS.       FIRST    SERIES. 


43 


TABLE  III. 

Ail  electromotive  forces  are  expressed  in  terms  of  thermo-element  W.  Initial 
pressure  287.5  mm-  0=0.003665.  Maximum  temperature  difference  between  middle 
and  either  end  of  bulb  0.5°.  Equation  used  for  "calculated"  electromotive  forces: 

«=  —305-5+8.  i749/+o.ooi654/-. 


No. 

Date. 

Observer 
electro- 
motive 
force.    Ele 
merit  IF. 

Calculated 
electro- 
motive 
force. 

Obs.- 
Calc. 

Obs 
Cal 

Constancy  of  standard  thermo- 
element  (W)  in  terms  of 
metal  melting-points. 

Zinc.       Silver.      Gold.  !  Copper. 

1907. 

Feb.    4 

0 

mv. 

mi'  . 

nn\ 

mv.        mv.        mv.       mv. 
•    34O3     • 

Mar.    6 

287:55'  ;;;; 

1 

6 

414 

49 

3367. 

3367 

i 

—  0 

.  1 

O 

.0    

2 

6 

414 

96 

337' 

337' 

5 

—  O 

•  5 

—  0 

.  i    

3 

6 

43' 

76 

3532.* 

3532 

4 

—  0 

4 

O 

.0    

4 

6 

432 

25 

3536.* 

3537 

—  I 

—  0 

.  i    

6 

287.39    .... 

5 

7 

419 

96 

34.6. 

34'9 

3 

—  3 

3 

—  o 

-3    

6 

7 

.    420 

oo 

34'7 

34'9 

7 

—  2 

7 

—  o 

7 

7 

436 

23 

357'-* 

3575 

4 

—  4 

4 

—  0 

•  5    

8 

7 

436 

40 

3574-* 

3577 

o 

-3 

o 

—  o 

•3    

9 

7 

458 

55 

3789  .* 

3790 

9 

9 

—  0 

10 

7 

45« 

94 

3793   * 

3794 

6 

—  i 

6 

—  o 

.2      \  

9 

287.34    .... 

1  1 

12 

408 

51 

3309  .* 

33'° 

o 

—  i 

o 

—  o 

.  1      :  

12 

12 

408 

•7  I 

2  2  1  n    * 

4312 

Q 

—  2 

Q 

—  o 

2 

'3 

12 

^<JO 

421 

/  * 

50 

JJ  l  u  • 

3430. 

3434 

I 

-4 

I 

—  o 

4    

'4 

12 

42: 

5° 

3429. 

3434 

I 

-5 

I 

—  o 

•5    

|  ^ 

j  2 

J52 

,x. 

:>  ZAO    * 

3  c  jo 

—  2 

_ 

—  o 

^ 

16 

12 

TV* 

•  432 

5 

89 

J  7TAJ  • 

354°  * 

3  7-t^ 

3543 

3 

-3 

/ 

3 

—  o 

•  j    

13 

287.35  .... 

'4 

.  .  .  F  3405    10461 

M3404                             10461 

>4 

287.35  .... 

>7 

15 

1078 

02 

10430.5 

10429 

I 

"  +  . 

4 

+  0 

.  i    

18 

«5 

1077 

05 

10419. 

10417 

7 

+  i 

3 

+° 

.  i    

15 

287.39  .... 

16 

i  0463 

10462 

'9 

20 

1013 

'7 

967,. 

9674 

7 

-3 

7 

—  0 

3    

20 

20 

.  .    IOI2 

45 

9664. 

9666 

i 

-2 

i 

—  o 

2      

21 

20 

1079.96 

10446. 

10451 

8 

-5 

8 

—  o 

5    

22 

20 

1078 

26 

10426. 

10431 

9 

—  5 

9 

—  o 

5    

23 

20 

1079 

O2 

10434. 

10440 

8 

-6 

8 

—  o 

6    

24 

2O 

1032 

75 

9895-5 

9901 

o 

-5 

5 

—  0 

5   :  

25 

2O 

1032 

70 

9895. 

9900 

4 

-5 

4 

—  0 

5    1 

21-22 

.  F  3405                        .    10457! 

M3405                               10456! 

23 

287.55    .... 

Apr.  2-3 

.  .   Fox)46    10214    I0454 

9046    10214    IO454 

22 

287.5!    .... 

26 

23 

416 

>4 

3383  ' 

3382 

8' 

"+° 

2 

0 

o    '  

2? 

23 

4'6 

26 

3384 

3384 

o 

0 

o 

o 

o    

28 

23 

520 

07 

4393 

4393 

3 

—  o 

3 

o 

0      

29 

23 

5'9 

7' 

4388. 

4389 

8 

-' 

8 

—  o 

2      '  

3« 

24 

.    489 

79 

4095. 

4095 

2 

—  o 

2 

o 

o    

31 

24 

489 

89 

4097. 

4096 

2 

+  0 

8 

+° 

1      

32 

24 

588 

oo 

5073   5 

5°73 

1 

+  0 

4 

0 

O      

33 

24 

587 

97 

5072   5 

5°72 

8 

—  o 

3 

0 

0  ,  ,  

*In  computing  the  constants  of  the  equation  at  the  head  of  Table  III,  the  observations  marked  *  were 
omitted  in  order  to  equalize  the  intervals  between  points. 


44 


HIGH   TEMPERATURE    GAS   THERMOMETRY. 
TABLE  III — Continued. 


No. 

34 
35 

36 

39 
40 
4i 

42 
43 
44 
45 

46 
49 

50 
51 
52 
53 
54 

$ 

57 

58 
59 
60 
61 
62 

i 

1 

69 
70 

7' 
72 
73 
74 

Date. 

Initial 
pressure 
in  bulb. 
(po) 

Observed 
tempera- 
ture gas 
thermom- 
eter. 

Observed 
elect!  o- 
motive 
force-.  Ele- 
ment W. 

Calculated 
electro- 
motive 
force. 

Obs.- 
Calc. 

Obs.- 
Calc. 

Constancy  of  sta 
element  (W) 
metal  melt 

Zinc.      Silver. 

ndard  thermo- 
in  terms  of 
ns-points. 

Gold.    Copper. 

1907. 

Apr.  24 
24 

y 

26 
26 

it 

26 

3 

27 
27 

29 
29 
29 
29 
29 

May  i-io 

13 
'3 
13 
'3 
'4 
>4 
•4 
'4 

16 

16 
16 
16 
16 
16 
16 

17 
'7 
>7 
'7 
'7 
'7 

18 
18 
18 
18 
18 
18 
19 
20-22 

23 

'287  '.56' 
287.55' 

688.44 
687.84 

550.81 
550.65 

866.45 
867.99 

644.34 
644.24 
957-20 
958.8. 

451.00 
450.76 
1057-32 
1057.09 

mv. 

6104. 
6099-5 

4700.5 

4698- 
6962. 

6957-5 
8020.5 
8029. 

5648. 
5646. 
9036. 
9054. 

3716. 
I  o  i  90  .  5 
10188. 

mv. 

6lo6.2 

6100.  o 

4699. 
4697. 
6964. 
6957. 
8019. 
8036. 

5648.5 
5647-5 
9034.8 
9053.0 

37.7.8 
37'5.5 
10186.7 
10184.  I 

mv. 

—  2.2 

-0-5 

—  O.2 
O.O 

mv. 

- 

mv.       mv. 

...1... 

+  '•4 
+0.5 

—  2.2 
0.0 

+  '-3 

-7-2 

-0-5 
-1-5 

+  1.2 
+  1.0 

+0.2 

+0.5 
+3-8 
+3-9 

+  +++++  ++++++  ++  1  1  1  ++  ++  1  1  I  I  I  :  :  +++  ++  1  1  1  +  1 

OOOOOO  OOOOOO  OOOOOOO  00000000  --0000  OOOO  OOOOOO 

;  | 
|  

F  3400 
M34oo 

9046  ; 
9050 

421.19 
422.06 
751-52 
751.36 
663.62 
663.33 
978.10 
978.82 

571-49 
571-37 
795-42 
795.46 
795.24 
1056.91 
1056.  15 

710.27 
710.70 
888.96 
889.50 
955-74 
955-01 

468.56 
468.94 
839  95 
839.46 
930.70 
930  .  62 

3430. 
3438.5 
6771.5 

6769-5 
5845.5 
5848- 
9274. 
9281. 

4907- 
4906. 
7242. 

7243- 
7240. 
10188. 
10177. 

6338. 
6341. 
8272. 
8277. 
9024. 
9015. 

3891. 
3894.5 
7730. 
7725. 
8741. 
8740. 

343  i  -  » 
3439  4 
6772.1 
6770.4 
5847-8 
5844.8 
9272.5 
9280.7 

4906.5 
4905-3 
7243-3 
7243-7 
7241.4 
10182.0 
.0173.1 

6335-2 
6339.7 
8268.5 
8274.5 
9018.2 
9009.9 

3888.0 

389'.7 
7727.8 
7722.4 
8735.4 
8734.5 

-0.9 
-0.6 
-0.9 

-+\l 

+  1-5 
+0.3 

+0.5 
+0.7 

-0-7 
-i-4 
+6.0 

+3-9 

+2.8 

+3:5 

+2-5 

+  5-8 
+  5-' 

&S 

+2.2 
+2.6 

+5-6 
+  5-5 

'287.'63' 

....... 

904010212  
904010216 

F3398 
M3398 

M9042 

GAS-THERMOMETER   MEASUREMENTS.      FIRST   SERIES. 

TABLE  IV. 

«= -305- 5+8. 1749 /+o. 001654 /-'. 


45 


Number  of 
observation. 

Observed 
temperature, 
gas  ther- 
mometer. 

Observed 
electro- 
motive force, 
element  »'. 

Calculated 
electro- 
motive force. 

Obs.- 
Calc. 

Obs.- 
Calc. 

i,  2,  5,  6,  13,  14, 

0 

me. 

mv. 

mv. 

0 

26,  27,  50,  51. 

418.97 

3407.7 

3409.9 

—  2.2 

—  O.2 

46,47 

450.88 

37'7-0 

3716.6 

+0.4 

0.0 

7'.  72 

468.75 

3892.8 

3889.9 

+2.9 

+  0.3 

30,  31 

489.84 

4096.0 

4095.7 

+0.3 

O.O 

28,29 

519.89 

4390  5 

4391.6 

—  i.i 

-0.  I 

36,  37 

550.73 

4699  3 

4698.3 

+  1.0 

+  0.1 

?8,  59 

571  43 

4906.5 

4905.9 

+0.6 

+  0.1 

32,  33 

587.99 

5073.0 

5073.0 

o.o 

o.o 

42,  43 

644.31 

5647  o 

5648.2 

—  I  .2 

—  0.  1 

54,  55 

663.48 

5846.8 

5846.4 

+0.4 

o.o 

34,  35 

688.14 

6101.8 

6103.  I 

-i   3 

-0.  I 

65,66 

710.49 

6339  5 

6337-5 

+2.0 

+0.2 

52,  53 

75'  -44 

6770.5 

677L3 

-0.8 

—  O.  I 

38,39 

769.,  8 

6959.8 

6960.9 

—  i.i 

-0.  I 

60,  61,  62 

795  37 

7241.7 

7242.8 

—  i.i 

—  O.  I 

73,74 

839.71 

7/27-5 

7725.1 

+2.4 

+0.2 

40,  41 

867.22 

8024.8 

8027.7 

-2.9 

-0.3 

67,68 

889.23 

8274-5 

8271.6 

+2.9 

+0.3 

75,  76 

930.66 

8740.5 

8734-9 

+  5-6 

+0.5 

44,  45,  69,  70 

956.68 

9032.2 

9028.9 

+3-3 

+0.3 

56,  57 

978.46 

9277.5 

9276.6 

+0.9 

+  0.1 

19,  20 

1012.80 

9667.5 

96/0.4 

-2.9 

-0.3 

24-  25 

1032.73 

9895.3 

9900.8 

-5-5 

-0.5 

48,  49,  63,  64 

1056.87 

10185.9 

lOl8l  .5 

+4-4 

+0.4 

17,  18,  21,  22,  23 

1078.31 

10430.0 

10432.5 

-2.5 

—  0.2 

In  their  paper  on  the  electromotive  force  of  metals  of  the  platinum  group, 
Holborn  and  Day1  have  shown  that  the  relation  between  the  thermo-electric 
force  and  the  temperature  could  be  represented,  within  wide  limits,  with 
an  accuracy  of  ±  1.0°  by  a  function  of  the  second  degree.  The  results  of 
our  measurements  are  represented  by  a  function  of  the  second  degree, 
between  400°  and  1 100°,  with  an  average  error  somewhat  less  than  0.2°,  the 
maximum  error  reaching  0.5°  in  two  cases. 

After  the  series  of  observations  represented  by  Tables  III  and  IV,  the 
bulb  was  evacuated  and  refilled  with  nitrogen  under  a  somewhat  higher 
initial  pressure,  £0=325  mm.  With  this  filling,  the  results  contained  in 
Table  V  were  obtained.  Column  5  of  this  table  contains  the  differences 
between  the  observed  electromotive  forces  and  the  electromotive  forces 
calculated  with  the  equation: 

e=  —265.6  +  8.0784  /+O.OOI7I24/2. 
The  average  difference  is  0.16°,  the  maximum  difference  0.5°. 


•Am.  Journ.  Sci.  (4),  8,  303-308.  1899. 


HIGH  TEMPERATURE   GAS   THERMOMETRY. 


TABLE  V. 

Thermo-couple  W.         Initial  pressure  325  mm. 
Equations  used  for  calculated  temperatures: 


0.003665. 


e=  —  265.6+8.07841+0.00171241* 
and  e  =  — 305.5+8.17491+0.0016541°  (last  column  parentheses). 


Date. 

Observed 
temperature, 
gas  ther- 
mometer. 

Observed 
electro- 
motive force 
element  W. 

,   Calculated 
electro- 
motive force 

Obs.- 
Calc. 

Obs.-Calc. 

1907. 

0 

mv. 

1 

mv. 

mv. 

0 

June  3  . 

482  .  i  o 

4025. 

4027.0 

—  2.O 

(+0.5)-0. 

482  .  i  5 

4026. 

4027.5 

'  •  5 

(+o.6)-o. 

581.29 

5009. 

5008.9 

+  0.1 

(+0.4)    o. 

582.22 

5017.5 

5018.2 

-0.7 

(+o.3)-o. 

675.07 

5974- 

5968.3 

+  5-7 

(+o.7)+o. 

June  4.. 

.      700.88 

6238.5 

6237.6 

+0.9 

(  +  0.2)+0. 

701.05 

6239. 

6239.4 

-0.4 

(+0.1)      0. 

June  6.  . 

.      772.10 

6992. 

6992.6 

-0.6 

(    o.o)  —  o. 

771.72 

6988. 

6988.5 

-0.5 

(    o.o)    o. 

May  29. 

.      819.00 

7495- 

7499.2 

-4.2 

(-o.4)-o. 

June  4.. 

.      860  .  52 

7954- 

7954  -o 

o.o 

(-0.2)     o. 

860.47 

7952- 

7953   5 

-'•5 

(-0.  l)-0. 

June  5 

908.47 

8488.5 

8486.7 

+  1.8 

(+0.2)+0. 

908.57 

8488.5 

8487.8 

+0.7 

(+0.l)+0. 

June  4.. 

955.30 

9016. 

9014.4 

+  1.6 

(+o.3)+o. 

954-9' 

901  1  . 

9010.0 

+  1.0 

(+0.2)+0. 

June  5  .  . 

995.32 

9474- 

947'  -4 

+  2.6 

(+o.5)+o. 

995.86 

9480. 

9477-6 

+2.4 

(  +  0.4)+0. 

June  6.  . 

.    1038.82 

9975- 

9974  3 

+0.7 

(+o.3)+o. 

1035.65 

9940. 

9937  5 

+2.5 

(+o.5)+o. 

1058.50 

10197.5 

10204.0 

-6.5 

(-o.3)-o.5 

The  agreement  between  this  series  of  observations  (Table  V)  and  the 
preceding  one  (Tables  III  and  IV)  is  also  remarkably  close.  Perhaps  this  is 
best  shown  by  the  fact  that  the  same  equation  used  for  the  "calculated" 
values  in  Tables  III  and  IV  will  represent  the  observations  of  Table  V  also 
with  ample  accuracy  for  purposes  of  interpretation.  These  differences  are 
carried  out  in  parentheses  in  the  last  column  as  an  added  evidence  of  the 
general  agreement  of  all  the  experimental  results.  It  will  be  remembered 
that  the  initial  gas  pressure  and  therefore  the  sensitiveness  of  the  instrument, 
together  with  all  the  correction  factors  which  depend  upon  it,  were  changed 
for  the  observations  of  Table  V. 


MELTING-POINTS  BASED  ON  FIRST  SERIES  (PT-IR  BULB). 

Four  metal  melting-points  were  used  to  fix  the  gas-thermometer  tempera- 
tures of  the  preceding  tables — zinc,  silver,  gold,  and  copper.  Analyses  of 
the  metals  are  given  on  page  85.  The  melting-point  determinations  (except 
gold,  of  which  the  only  sample  then  available  accidentally  became  contam- 
inated with  iron  during  the  observations  and  could  no  longer  be  used  as  a 
standard)  are  contained  in  Table  VI. 


(JAS-THERMOMETER    MEASUREMENTS.      FIRST   SERIES. 


47 


TABLE  VI. — TEMPERATURE  OF  FIXED  POINTS. 

FIRST    SERIES,    PLATIN-IRIDIUM    BULB. 
ZINC. 


FjementIF     Gas-ther- 

H  •    Observed 

*<fromer            Date.           '"nfo^er"     tem^er^          ^n"i"c 

tempera- 
ture of  melt- 

furnace.            ture. 

ing  zinc. 

1907.              mv.                               mv. 

i 
° 

I            Mar.    6         3367.          414  .49       3405. 

4.8.5 

2           Mar.    6         3371.         414  .96       3405. 

4.8.5 

5           Mar.    7         3416.         4  19  .96       3405. 

418.8 

6           Mar.    7          3417           420.00       3405. 

4.8.7 

13           Mar.  12          3430           421.30       3405. 

418.9 

14           Mar.  12         3429.         421.5°       3405 

419.0 

26           Apr.  23         3383.         416.14       3405 

4.8.4 

27           Apr.  23         3384.         416.26       3405. 

4.8.4 

50           May  13         3430.         421.19       3400 

4.8.0 

51           May  13         3439-         422.06       3400. 

418.0     i 

Melting-point  of  zinc  (Pt-Ir  bulb)—  Mean  

....     4'8.5° 

Average  error  

0.3° 

SILVER. 

44           Apr.  27         9036.          957-20       0045. 

958.0 

45            Apr.  27         9054.          958.81        9045. 

958.1 

56           May  14         9274.          97810       9045. 

958.1 

57           May  14         9281.         978.82       9045. 

958.2 

69           May  17         9024.          955  74       9042 

957  3 

70           May  17         9015.          955  o.        9042. 

957-4 

June     4         9016.          955  .30       9042 

957.6 

June    4         9011.          9549'        9042. 

957-6 

[can  

957-8° 

verage  error  

0.3 

pproximate  correction  for  loss  of  heat  at  ends  of  bulb     +2  .  ° 

Melting  point  of  silver  (Pt-Ir  bulb)  

960° 

COPPER. 

17           Mar.  15        10430.5      1078.02      10461 

1080.5 

.8           Mar.  1  5        10419.        I077-°5      10461 

1080.5 

21           Mar.  20       10446.        1079.96     10456 

i  080  .  9 

22           Mar.  20       10426.        1078.26      10456 

1080.8     i 

23           Mar.  20       10434.        1079.02      10456 

1080.9 

lean  

1080.7° 

verage  error  

0.2° 

approximate  correction  for  loss  of  heat  at  ends  of 

bulb.      +2.    ° 

Melting-point  of  copper  (Pt-Ir  bulb) 


1083' 


48  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

This  work  with  the  platin-iridium  bulb  developed  two  serious  limitations 
which  have  received  detailed  consideration  and  correction  elsewhere  (pp. 
50  and  51).  They  are  these: 

(1)  Independent  end  coils  wound  on  the  furnace  tube  (see  Fig.  i)  can  be 
made  to  give  perfectly  uniform  temperature  distribution  along  the  sides  of 
the  bulb  from  end  to  end,  but  not  on  the  end  surfaces. 

(2)  The  sublimation  of  the  iridium  in  the  bulb,  while  not  prohibitive  below 
1 1 00°,  affects  the  homogeneity  of  thermo-elements  during  the  exposure 
necessary  to  calibrate  them,  at  temperatures  above  900°,  to  an  extent  which 
greatly  limits  their  subsequent  usefulness. 

This  second  limitation  led  to  the  substitution  of  a  bulb  containing  rho- 
dium in  place  of  iridium,  but  does  not  seriously  affect  the  results  already 
obtained  with  the  platin-iridium  bulb,  which  were  all  below  1100°  (Tables 
III,  IV,  V).  The  first  limitation,  on  the  other  hand  (the  cooling  of  the  end 
surfaces  of  the  bulb  by  radiation),  was  not  detected  until  the  observations 
with  the  platin-iridium  bulb  were  completed  and  published  and  the  bulb 
itself  was  returned  to  the  maker.  It  is  therefore  no  longer  possible  to  supply 
a  correction  factor  made  with  the  same  bulb  under  identically  the  same  condi- 
tions with  which  to  compensate  the  error  in  the  final  temperatures  of  Table 
VI.  The  new  platin-rhodium  bulb,  however,  except  for  the  reentrant  tube 
below,  was  of  the  same  dimensions  as  the  old  one  and  the  furnace  conditions 
were  very  closely  reproducible.  A  measurement  was  accordingly  made  with 
the  new  bulb  as  nearly  as  possible  under  the  conditions  previously  employed 
and  another  with  a  diafram  in  the  tube  to  cut  off  and  reflect  back  the  radi- 
ation going  out  from  the  upper  end  of  the  bulb.  The  temperature  was  that 
of  melting  copper  (1082.6°).  This  diafram  above  the  bulb  raised  the  gas 
thermometer  temperature  0.9°.  A  similar  test  for  the  effect  of  radiation 
from  the  bottom  could  not  be  made,  but  it  was  probably  of  similar  magni- 
tude. The  total  error  incurred  in  the  above  measurements  through  the 
radiation  from  the  ends  of  the  bulb  would  therefore  be  of  the  order  of  mag- 
nitude 2°  at  the  copper-point.  At  low  temperatures  the  effect  is  too  small 
to  make  it  worth  while  estimating  a  correction  factor.  If  we  then  suppose 
the  radiating  power  of  the  20  per  cent  platin-rhodium  alloy  to  be  comparable 
with  that  of  the  iridium  alloy,  one  may  apply  this  correction  to  the  mean 
temperatures  obtained  with  the  platin-iridium  bulb  as  given  in  Table  VI. 
While  these  values  are  in  good  agreement  with  the  later  results  obtained 
with  the  platin-rhodium  bulb,  they  are  not  included  in  Table  XIX  because 
of  the  approximative  character  of  this  ex-post-facto  correction. 

10.     INTRODUCTION  TO  THE  SECOND  SERIES,  INCLUDING  THE 
HIGHER  TEMPERATURES,   1100°-1 600°. 

Above  1 1 00°  considerable  uncertainty  has  existed  in  the  temperatures  of 
various  fixed  points.  The  melting-point  of  nickel,  determined  by  extra- 
polation from  the  data  of  Holborn  and  Wien  (I4840),1  has  been  frequently 
employed.  The  curve  of  the  platinum-platinrhodium  thermo-element, 
extrapolated  beyond  the  copper-point,  has  been  still  more  generally  used, 
but  like  most  extrapolations,  may  lead  to  quite  erroneous  results.  The 
only  gas-thermometer  comparison  that  has  been  made  in  this  region  is  that 

'Holborn  and  Wien.  Wied.  Ann.  47,  107-134,  1892;  and  56,  360-396,  1895. 


INTRODUCTION    TO    SECOND    SERIES.  49 

of  Holborn  and  Valentiner,1  but  by  their  own  estimate  the  accuracy  of  the 
upper  portion  of  their  scale  is  not  greater  than  ±  10°.  The  chief  purpose 
of  our  work  was,  therefore,  to  establish  the  temperature  of  several  fixed 
points  between  1100°  and  1600°  and  to  ascertain  what  curve  is  followed 
by  the  platinum-platinrhodium  thermo-element  in  this  region,  with  an 
accuracy  comparable  to  that  obtained  in  the  lower  portion. 

The  plan  of  procedure  is  simple.  It  consists,  first,  in  selecting  certain 
fixed  thermometric  points,  usually  melting-points  of  metals,  and  in  deter- 
mining their  reproducibility ;  second,  in  making  a  measurement  of  the  true 
temperature  on  the  nitrogen  scale  at  or  close  by  these  fixed  points;  third, 
in  transferring  these  known  temperatures  by  means  of  a  thermo-element 
over  to  the  fixed  points  selected.  This  transference  by  the  thermo-element 
is  necessary  because  the  thermometer  bulb  can  not  be  put  directly  into 
melting  or  solidifying  substances  at  high  temperatures.  The  relation  of 
electromotive  force  to  temperature  for  any  particular  kind  of  thermo- 
element does  not  enter  into  the  problem  when  the  temperatures  measured 
are  close  to  the  fixed  points;  a  linear  correction  is  then  abundantly  accurate. 
The  interpolation  curve,  for  any  element,  between  the  fixed  points  estab- 
lished by  the  gas  thermometer  is  therefore  a  separate  matter. 

No  other  method  of  transferring  the  gas-thermometer  temperatures  can 
be  employed  in  this  region.  Of  the  two  methods  of  comparative  temper- 
ature measurement  in  common  use,  one,  the  platinum  resistance  pyrometer, 
can  not  be  used  above  1 100°  with  certainty — the  other,  the  radiation  pyro- 
meter, is  of  wholly  inadequate  sensitiveness  in  any  of  the  forms  hitherto 
devised. 

The  questions  which  remain  to  be  answered  are,  then:  (i)  How  exact 
and  uniform  can  the  temperature  of  the  gas  in  the  bulb  be  made  (inde- 
pendently of  any  effort  to  measure  this  temperature)  ?  (2)  How  accurately 
can  its  pressure  be  measured  in  order  to  establish  that  temperature  on  the 
nitrogen  scale?  (3)  How  accurately  can  this  temperature  be  transferred 
from  the  thermometer  and  compared  with  the  fixed  melting-point?  (4) 
How  accurately  can  the  fixed  points  be  reproduced  for  purposes  of  calibra- 
tion of  secondary  measuring  devices? 

As  has  been  stated,  our  experience  has  convinced  us  that  most  of  the 
variations  in  the  gas-thermometer  temperatures  of  the  fixed  points  com- 
monly used  by  various  observers,  are  due,  not  to  differences  in  the  proper- 
ties of  different  gases,  nor  to  differences  in  pressure,  nor  to  differences 
between  the  constant-volume  and  constant-pressure  scales,  all  of  which 
have  been  frequently  discussed  from  the  theoretical  standpoint;  but  to 
systematic  errors  residing  in  the  apparatus  and  the  methods  employed. 
A  large  portion  of  the  present  work  has  therefore  been  devoted  to  finding 
out  experimentally  the  effect  of  variations  in  all  those  conditions  which 
might  affect  the  results  systematically. 

'Ann.  d.  phys.  (4).  22,  1-48,  1907. 


50  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

11.    CHANGES  IN  THE  APPARATUS. 

In  all  essentials  the  gas-thermometer  apparatus  used  for  this  second  series 
of  measurements  is  identical  with  that  already  described,  except  for  the  bulb. 

THE  PLATIN-RHODIUM  BULB. 

Primarily  and  obviously,  the  bulb  must  be  able  to  hold  the  expanding 
gas  without  absorbing  or  losing  any  portion  of  it  throughout  the  temper- 
ature range  of  the  measurements.  A  secondary  requirement,  the  import- 
tance  of  which  increases  rapidly  when  high  accuracy  is  sought,  is  that  it 
shall  be  possible  to  use  several  thermo-elements  in  the  furnace  with  the  bulb 
without  their  readings  being  endangered  by  contamination  from  the  bulb 
material.  As  long  as  this  intermediary  role  of  the  thermo-element  remains 
a  necessary  one  and  alloys  of  platinum  continue  to  provide  the  only  suc- 
cessful bulb  material,  the  contamination1  of  the  platinum  wire  of  the  element 
by  the  alloyed  iridium  or  other  platinum  metal  in  the  bulb  will  remain  a 
serious  consideration  in  all  temperature  measurement  above  900°. 

Although  the  platin-iridium  bulb  served  well  as  a  gas-container,  the 
contaminating  effect  of  the  iridium  upon  the  thermo-elements  made  the 
life  of  the  latter,  for  measurements  of  this  degree  of  accuracy,  very  short. 
Accordingly,  at  the  close  of  the  first  series  of  experiments,  a  change  was 
made  from  the  platin-iridium  bulb  to  one  of  platin-rhodium  (80  parts  plati- 
num, 20  parts  rhodium)  160  mm.  long  and  47  mm.  in  diameter.  Inasmuch 
as  one  of  the  wires  of  the  thermo-element  itself  contains  10  per  cent  of 
rhodium  to  which  the  platinum  wire  is  always  exposed  (and  which  gradu- 
ally contaminates  it,  too,  although  very  slowly2),  it  was  thought  that  the 
substitution  of  a  rhodium  alloy  in  the  bulb  might  serve  to  retain  the  neces- 
sary qualities  of  stiffness  and  regularity  of  expansion  with  a  minimum  of 
disadvantage  in  the  matter  of  contamination.  These  expectations  have  been 
completely  realized.  Although  the  rhodium  alloy  is  less  rigid  at  tem- 
peratures of  1000°  and  beyond  than  the  iridium  alloy,  and  requires  more 
careful  adjustment  for  equal  pressure  within  and  without,  no  sagging  of  the 
bulb  walls  or  deformation  from  the  gas  pressure  has  appeared  up  to  1550°. 
Meanwhile,  the  contamination  of  the  thermo-elements  in  the  presence  of 
the  rhodium  alloy  is  now  reduced  to  about  20  per  cent  of  its  former  magni- 
tude for  a  given  temperature  and  time  of  exposure. 

THE  FURNACE. 

The  common  practice  of  recent  observers  (Callendar,  Harker,  Holborn 
and  Day,  Jaquerod  and  Perrot)  has  been  to  use  a  cylindrical  bulb  in  which 
the  length  was  three  or  four  times  the  diameter,  inclosed  in  a  concentric 
furnace  tube  (air-bath)  heated  by  the  electrical  resistance  of  a  coil  of  wire 
wound  upon  or  within  it.  To  this  bulb  the  heat  is  applied  radially  over  its 
cylindrical  surface,  but  no  heat  is  supplied  at  the  ends.  The  furnace  tube 
itself  and  the  winding  of  the  coils  have  been  changed  at  different  times  and 
in  a  variety  of  ways  in  order  to  vary  the  distribution  of  the  heat  supply. 
The  arrangement  which  has  become  usual  with  us  is  to  wind  the  coil  some- 
what more  closely  at  the  ends  than  the  middle,  with  the  idea  of  providing 

'For  a  detailed  account  of  the  behavior  and  treatment  of  contaminated  thermo-elements.  see  Walter  P. 
White,  Phys.  Rev.,  23,  449-474,  1906. 

"White,  "  Constancy  of  thermo-elements."  Phys.  Rev.,  23,  463-465.  1906.  Phys.  Zeitschr.,  8,  331-333 , 
1907. 


CHANGES   IN   THE    APPARATUS.  51 

partial  compensation  for  the  inevitable  heat  losses  at  the  ends  of  the  fur- 
nace in  this  way,  and,  in  addition,  to  insert  supplementary  coils  of  smaller 
wire  in  the  ends  of  the  furnace  tube  in  order  to  provide  a  small,  independ- 
ently regulated  heat  supply  which  can  be  superposed  upon  that  of  the  main 
coil  and  give  the  desired  uniformity  at  any  temperature  likely  to  be  em- 
ployed. A  furnace  tube  arranged  in  this  way,  except  for  accidental  varia- 
tions, caused,  for  example,  by  the  flaking  off  of  the  furnace  lining,  affords 
uniform  temperature  distribution  over  a  length  of  20  cm.  in  the  center  of 
the  tube  for  a  range  of  temperature  from  300°  to  1550°,  and  no  one  tem- 
perature is  more  difficult  to  regulate  than  another.  This  arrangement 
contains  a  limitation,  however,  of  considerably  greater  magnitude  than  was 
at  first  suspected.  The  ends  of  the  bulb  face  the  comparatively  cold  ends 
of  the  furnace  tube  and  radiate  a  sufficient  quantity  of  heat  toward  these 
cold  ends  to  reduce  the  temperature  of  the  end  surfaces  of  the  bulb  some 
6°  or  8°  below  the  mean  temperature  of  the  cylindrical  surface. 

This  may  appear  to  be  a  rather  obvious  condition  to  be  overlooked,  but 
it  is  a  common  practice  of  physicists  to  assume  that  the  temperature  is  con- 
stant over  a  radial  cross-section  near  the  center  of  a  cylindrical  furnace 
which  is  reasonably  long  in  comparison  with  its  diameter.  With  this  in 
mind,  the  probability  is  even  greater  that  a  metallic  conductor  only  4  cm. 
in  diameter  (the  end  surface  of  the  bulb),  perpendicular  to  the  axis  in  such  a 
furnace,  will  have  a  uniform  temperature  between  its  center  and  periphery. 
The  fact  is  that  neither  of  these  assumptions  is  justified,  even  in  furnaces 
as  long  as  twenty  times  the  diameter.  This  was  shown  in  a  number  of 
actual  measurements  made  in  different  furnaces  under  varied  conditions, 
differences  of  several  tenths  of  a  degree  being  found  as  low  as  300°,  and  of 
several  degrees  at  1000°  and  higher. 

This  situation  demonstrates  the  futility  of  depending  upon  metallic 
conductivity  (of  platinum)  to  equalize  a  steep  temperature  gradient,  and 
again  emphasizes  the  fact  that  the  air-bath,  or,  more  explicitly,  the  tem- 
perature distribution  within  the  heating  chamber,  is  the  most  uncertain 
factor  remaining  in  gas  thermometry. 

On  account  of  difficulties  in  manipulation  and  accidental  leakage  into 
the  thermo-element  system,  we  preferred  not  to  introduce  more  heating  coils 
into  the  furnace  tube  and  therefore  undertook  to  stop  the  loss  of  heat  by 
inserting  thin,  platinum-covered  diaf  rams  opposite  the  ends  of  the  bulb.  The 
situation  was  still  further  safeguarded,  in  exchanging  the  platin-iridium  for 
the  platin-rhodium  bulb,  by  adding  a  reentrant  tube  at  the  lower  end  of  the 
bulb,  to  enable  us  to  measure  the  actual  temperature  prevailing  at  its  center 
as  well  as  over  the  surface.  We  thought  thus  to  obtain  a  more  representa- 
tive integral  of  the  surface  temperature  and  a  competent  comparison  of  this 
integral  with  the  temperature  actually  prevailing  at  the  center  of  the  bulb. 

Only  one  change  was  made  in  the  manometric  apparatus.  The  gas, 
instead  of  being  introduced  through  the  three-way  cock  at  the  bottom  of 
the  short  arm  of  the  manometer,  which  necessitated  its  bubbling  through 
the  mercury,  was  admitted  by  a  stopcock  and  a  slanting  side  tube  blown 
into  the  manometer  tube  about  30  cm.  below  the  fixed  point. 

Changes  in  the  expansion-coefficient  apparatus  are  described  on  p.  61. 


52  HIGH  TEMPERATURE   GAS  THERMOMETRY. 

12.   DETAILS,  ERRORS,  AND   CORRECTIONS. 

The  gas  thermometer  for  very  high  temperatures  has  now  reached  a  stage 
of  development  where  it  becomes  necessary  to  examine  many  small  sources 
of  error.  These  will  be  discussed  in  the  succeeding  paragraphs  without 
attempting  to  classify  separately  the  variable  errors  of  observation,  and  the 
systematic  errors  which  may  arise  from  conditions  of  the  measurements  or 
from  constant  corrections. 

To  bring  out  the  plan  of  investigation  of  these  errors,  it  will  be  well  to 
recall  the  derivation  of  the  gas-thermometer  formula.  The  gas  scale,  as 
is  well  known,  is  defined  by  the  relation 


in  which  K  and  a  are  constants  and  t  is  a  function  of  the  product  pv,  p 
and  v  being  the  pressure  and  volume  of  a  fixed  mass  of  the  gas.  K  and  a 
are  determined  by  two  further  conventions: 

When  p=  p0  and  v=  v0  (at  the  melting-point  of  ice),  t=o  (2) 

When  p=pIOO  and  V=VIOQ  (at  the  boiling-point  of  water),  t=  100          (3) 
It  is  then  evident  that 


ioo/>0 

which  defines  a  as  the  mean  pressure-coefficient  of  the  gas  between  o°  and 
1  00°  (when  vIOO  and  v0  are  nearly  equal)  ;  and 

K=p0v0 
The  temperature  /  is  therefore  defined  by  the  formula 

P~Po  (4) 


ap0 

the  scale  depending  upon  the  gas  chosen,  the  value  of  p0,  and  the  ratio  —  . 

Ro 

In  the  theoretical  constant-volume  thermometer  this  ratio       is  unity,  but 

^o 

in  the  experimental  constant-volume  thermometer  it  always  varies  con- 
siderably from  i.    We  have  therefore  preferred  to  treat  equation  (4)  as 

the  fundamental  equation,  introducing  in  place  of     ,  however,  the  proper 

V0 

function  of  the  expansion  coefficient  of  the  bulb  material. 

Since  apparatus  designed  for  high-temperature  work  is  not  suited  for 
the  most  accurate  determination  of  a,  a  has  been  treated  in  this  discussion 
as  a  separately  determined  constant. 

In  the  experimental  gas  thermometer  there  is  always  a  small  space  in 
the  tube  connecting  with  the  manometer,  and  this  space  is  at  various  tem- 
peratures other  than  t.  The  pressure  (p'  or  p0')  actually  measured  is  not, 


DETAILS,  ERRORS,  AND   CORRECTIONS.  53 

therefore,  the  p  or  p0  of  the  formula.  Imagine  that  this  supplementary 
space  is  heated  up  to  the  uniform  temperature  /,  without  any  change  in 
its  volume,  and  let  the  resulting  corrected  pressure  be  p  (or  p0  as  the  case 
may  be).  Furthermore,  let 

V   =  volume  of  bulb  at  /°. 

V0  =  volume  of  bulb  at  o°. 

vl  =  volume  of  unheated  space,  which  is  at  temperatures  other  than  / 
(or  than  o°). 

/,    =  temperature  of  this  space. 

13     =  linear  expansion  coefficient  of  the  bulb  material. 

Under  these  conditions,  formula  (4)  becomes  : 


Since 

T/4-  V+V        I+^ 

f+J  =  £     =  y~~=I  + 

the  formula  for  t  becomes 

r,  /'. ,  3#  \   .1 

(5) 
In  this  formula     '  is  a  very  small  correction  term;  while  the  important 

'0 

quantities  to  be  measured  are  p0,  p,  a,  and  8.     The  ratio    ^   becomes  of 

'  o 

importance,  however,  in  determining  the  corrected  pressure  p  from  the 
measured  pressure  p'.    The  derivation  of  this  correction  is  as  follows: 

The  mass  of  the  gas  in  the  unheated  volume  under  the  actual  conditions 

./ 

of  measurement  is  proportional  to  -\—.  ;  the  mass  of  the  gas  within  the 

I  -f-CUf 

p'V 

bulb  is  proportional  to   — — .    If  we  now  suppose  the  unheated  space  raised 

i -ral- 
to  the  uniform  temperature  /  without  change  of  volume,  the  pressure  being 


\ 
thereby  raised  to  p,  the  total  mass  is  proportional  to        ,     /  •       Therefore, 


t. 

i  +  a/,      l+a*         i  +  a/ 
whence 


»f          a/-a/A 

V+VI  •  I+a/J 


This  correction  is  easily  calculated  and  tabulated;  or,  better,  the  factor 
in  parenthesis  (in  the  second  member  of  the  equation)  is  plotted  against 
temperature.  In  practice,  the  volume  i',  is  divided  into  three  portions  at 
temperatures  //,  t/',  and  f,'"  as  explained  on  page  58,  and  the  correc- 


54  HIGH   TEMPERATURE   GAS  THERMOMETRY. 

tions  obtained  from  the  curve  for  each  of  these  portions  are  simply  added 
together  to  obtain  the  total  correction  p  —  p'.  With  these  corrected  pres- 
sures, p0  and  p,  the  temperature  /  is  calculated  by  formula  (5)  on  page  53. 

This  method  of  computation  yields,  of  course,  the  same  values  for  t  as 
the  formula  on  page  40,  but  has  the  advantage  of  showing  more  clearly  the 
magnitude  of  the  corrections. 

The  discussion  of  errors  and  corrections  will  now  be  taken  up  under  the 
general  outline  sketched  on  page  49. 

TEMPERATURE  OF  THE  GAS  IN  THE  BULB. 

Uniformity. — Above  the  temperatures  where  a  liquid  or  vapor  bath 
can  be  used  to  secure  uniformity,  the  differences  of  temperature  between 
different  parts  of  a  furnace  have  always  been  a  serious  limitation  to  the 
accuracy  of  the  gas  thermometer.  This  variation,  even  in  a  furnace  con- 
taining well-conducting  materials,  is  much  larger  than  has  usually  been 
assumed,  and  the  three  equalizing  factors  of  conductivity,  radiation,  and 
convection  by  air-currents  are  all  credited  with  much  greater  influence  in 
bringing  about  uniformity  than  they  really  possess.  It  sometimes  happens 
that  our  faith  in  these  factors  is  inversely  proportional  to  our  quantitative 
information  about  them. 

In  the  first  measurements  with  the  new  bulb,  the  end  elements  were 
placed  on  the  axis  of  the  bulb,  in  positions  i  and  9  (Fig.  8),  instead  of  on 
the  periphery  of  the  cylinder.  It  became  evident  at  once  that  the  support- 
ing tube  in  the  bottom  of  the  furnace,  used  in  earlier  measurements,  had 
a  considerable  cooling  influence  on  the  central  portion  of  the  bottom,  an 
effect  which  would  tend  to  make  the  results  low.  This  effect  was  largely 
obviated  by  replacing  the  heavy  magnesite  tube  (Fig.  i,  p.  18)  with  a  thin 
Marquardt  porcelain  tube,  in  the  top  of  which  was  placed  a  Marquardt 
crucible,  cut  out  into  a  three-pronged  support  (Fig.  9,  p.  56).  The  bottom 
of  the  crucible  acted  as  a  screen  to  prevent  radiation  from  the  bottom  of 
the  bulb,  and  the  smaller  thickness  and  thermal  conductivity  of  the  tube 
practically  prevented  the  loss  of  heat  from  the  bottom  by  conduction. 
Later,  a  second  diafram  was  added  about  i  cm.  lower  down,  primarily  for 
the  purpose  of  centering  the  tube  and  bulb  in  the  furnace,  but  without 
noticeable  effect  on  the  temperature  distribution. 

In  addition  to  the  three  thermo-elements  mentioned,  a  fourth  was  located 
inside  the  reentrant,  in  position  8.1  Several  trials  under  varied  conditions 
confirmed  the  fact  that  this  element,  when  the  other  three  were  set  equal, 
was  2°  to  3°  hotter  than  the  one  on  the  outside.  A  thorough  exploration 
of  the  distribution  of  temperature  over  the  surface  of  the  bulb  was  therefore 
undertaken. 

Since  the  number  of  wires  which  could  be  led  out  through  the  packed 
joints  was  limited,  the  plan  was  adopted  of  using  the  bulb  itself  as  a  differ- 
ential thermo-element,  single  platinum  wires  being  tied  to  the  bulb  at  points 
whose  temperature  was  to  be  determined.  Each  of  these  wires  formed, 
with  the  platinum  of  the  standard  element  tied  to  the  bulb  at  the  middle, 
a  differential  element  which  would  read  zero  if  the  wires  were  alike  and  if 
no  difference  of  temperature  existed  between  the  two  points  on  the  bulb. 

'See  Fig.  8,  and  note,  p.  55. 


DAY  AND  SOSMAN 


7 — : 


"7SV 


FIG.  8.  Vertical  section  of  the  gas  thermometer  bulb,  and  photograph 
made  after  the  palladium-point  determination  showing  all  the  elements  and 
the  diaframs  in  position.  The  numbers  are  used  in  the  tables  of  data  to 
designate  the  positions  of  the  thermo-elements  grouped  about  the  bulb. 


DETAILS,  ERRORS,  AND   CORRECTIONS.  55 

The  relation  of  the  wires  was  established  by  sealing  each  in  turn  to  the 
platinum  of  the  standard,  and  measuring  their  E.  M.  F.  at  various  tempera- 
tures. The  readings  varied,  according  to  the  quality  of  the  wire,  from  o 
to  40  microvolts.  The  method  of  evaluating  differences  of  temperature, 
when  such  existed,  is  discussed  on  page  66. 

The  distribution  of  temperature  lengthwise  of  the  bulb  was  first  investi- 
gated, and  auxiliary  wires  were  placed  at  the  levels  i  (base  of  stem),  2  (top 
shoulder),  6  (bottom  shoulder),  7  (bottom,  outside  of  funnel),  in  addition  to 
thermo-elements  at  4  (middle  outside),  8  (inside  reentrant),  and  9  (bottom, 
just  inside  of  funnel).1 

With  this  system  of  thermo-elements,  it  was  found  that  at  1082°,  when 
9  was  brought  to  equality  with  4  and  i,  then  at  the  bottom  of  the  bulb 
element  6  was  superheated  6°  to  8°  and  element  7  about  4°,  due  entirely 
to  the  fact  that  the  thermo-element  at  9,  not  being  in  contact  with  the  bulb, 
lost  sufficient  heat  by  conduction  and  radiation  downward  to  keep  its 
temperature  below  that  of  the  metal  surrounding  it.  The  element  8,  on 
the  other  hand,  received  heat  by  conduction  up  the  reentrant  tube  and 
by  radiation  from  below,  which  made  it  read  higher  than  the  element  at 
the  same  level  outside.  The  element  at  position  9  was  therefore  discarded 
and  each  setting  of  temperature  was  made  with  the  elements  which  were 
attached  directly  to  the  bulb,  i.  e.,  by  bringing  i,  4,  and  7,  or  2,  4,  and  6  to 
uniform  temperature.  In  fact,  at  the  highest  temperatures  where  the  danger 
of  unequal  distribution  was  greatest,  both  arrangements  were  employed  in 
successive  measurements  at  each  temperature. 

The  temperature  between  the  middle  and  the  top  shoulder  was  also 
examined  in  several  experiments.  The  temperature  at  this  position  was 
found  to  be  within  0.5°  of  the  other  two,  with  a  tendency  to  be  lower  than 
these. 

Further  experiments  showed  that,  in  addition  to  the  possibility  of  vertical 
variation  of  temperature,  there  was  a  variation  around  the  circumference 
of  the  bulb.  This  amounted  in  the  worst  case  (at  1450°)  to  a  variation  of 
1.3°  from  the  mean,  four  elements  being  used  around  the  circumference  to 
make  the  test.  This  variation  seemed  to  be  due  either  to  unequal  conduc- 
tivity of  the  furnace  material  at  different  points  or  to  the  flaking  off  of 
small  portions  of  the  furnace  lining,  leaving  exposed  places  on  the  wire. 
Variations  of  this  character  are  probably  an  unavoidable  result  of  using 
a  furnace  where  the  heat  supply  is  so  near  to  the  point  where  it  is  measured, 
as  is  the  case  with  the  furnace  coil  which  is  wound  on  the  inside  of  the  tube. 
This  form  of  winding  is  necessary,  however,  in  order  to  reach  the  highest 
temperatures,  so  that  absolute  uniformity  of  temperature  around  the  bulb 
was  sacrificed  to  the  increased  range  of  the  instrument. 

After  this  variation  was  discovered,  measurements  were  always  made 
with  four  elements  at  equal  distances  around  the  circumference  of  the  bulb 
and  the  mean  of  their  readings  was  taken. 

In  order  to  be  perfectly  certain  that  no  systematic  error  was  being  intro- 
duced by  using  this  one  form  of  furnace  (Fig.  9)  throughout,  it  was  replaced 

'The  system  of  numbering  the  positions  of  elements  on  the  bulb  is  shown  in  Fig.  8.  The  figure  before 
the  decimalpoint  indicates  the  horizontal  level,  the  figure  after  the  decimal  indicates  the  orientation  around 
the  bulb.  For  instance,  an  element  in  position  3.5  would  be  about  half-way  between  the  top  and  middle 
and  on  the  side  of  the  bulb  away  from  the  front  of  the  apparatus. 


HIGH   TEMPERATURE   GAS   THERMOMETRY. 


temporarily  by  a  furnace  of  platinum  wire  wound  on  the  outside  of  a  similar 
tube.  In  this  way  a  heavy  mass  of  good  heat-conducting  material  was 
introduced  between  the  source  of  heat  and  the  bulb,  with  the  expectation 
that  a  more  uniform  temperature  might  thereby  be  obtained  in  the  inside 
space.  The  two  types  of  furnace  are  shown  in  Figs.  9  and  10. 

A  measurement  at  the  copper  point  with  the  outside-wound  furnace  gave 
as  the  melting-point  of  copper  1082.6°,  a  value  identical  with  the  mean  of 
the  results  obtained  with  the  other  furnace,  thus  proving  that  no  systematic 
error  need  be  feared  from  the  inside- wound  type  of  furnace.  The  horizontal 
uniformity  obtained  in  the  outside-wound  furnace  was  better  than  that  in 
FIG.  9.  FIG.  10. 


FIG.  9.  Section  of  furnace  and  bulb  showing  an  arrangement  of 
coils  and  diaframs  about  the  bulb  which  gave  a  most  uniform  tem- 
perature distribution  in  the  measurement  of  both  high  and  low 
temperatures.  The  supplementary  end  coils  were  independently 
heated  and  regulated. 

FIG.  10.  A  special  arrangement  of  the  heating  coil  and  diaframs 
designed  to  give  a  very  uniform  temperature  distribution  about 
the  bulb.  The  coil  was  heavily  ballasted  inward  with  a  good  heat 
conductor  and  outward  with  a  poor  conductor.  The  heating  coil 
was  also  divided  into  three  sections  which  could  be  independently 
regulated.  This  furnace  was  used  at  the  copper  point  only. 

the  inside-wound,  but  the  furnace  was  more  difficult  to  regulate  and  to 
hold  at  a  given  temperature. 

Constancy  of  conditions. — Several  causes  interfered  with  the  establish- 
ment of  a  constant  temperature  for  observation.  The  three  heating  currents 
required  constant  observation  and  readjustment  with  the  gradual  extension 
of  the  heated  zone  toward  the  outside  of  the  furnace.  This  came  to  equi- 
librium for  a  particular  temperature  after  about  half  an  hour,  after  which 
the  bulb  was  held  steady  15  to  30  minutes  longer  before  readings  of  the 
pressure  were  taken.  The  temperature  thus  established  could  be  relied  upon 
to  remain  constant  to  within  i  to  3  microvolts  (0.1°  to  0.3°)  during  the 
course  of  the  pressure  measurements. 

Above  1100°  a  noticeable  leakage  of  current  from  the  heating  coil  into 
the  bulb  and  thermo-elements  frequently  appeared.  This  may  have  been 


DETAILS,  ERRORS,  AND   CORRECTIONS.  57 

due  in  part  to  conductivity  across  the  narrow  air-space  between  bulb  and 
coil,  but  was  probably  chiefly  due  to  accidental  contact  between  the  pro- 
tecting tube  of  one  of  the  thermo-elements  and  the  furnace  wall.  To  obviate 
any  uncertainty  from  this  cause,  it  was  found  necessary  to  use  alternating 
current  for  all  temperatures  above  1100°.  This  was  less  easy  to  regulate 
than  the  direct  current  from  storage  batteries,  but  by  careful  regulation 
of  the  voltage  of  the  motor  generator  supplying  the  alternating  current, 
equally  satisfactory  results  were  obtained. 

The  constancy  and  exactness  of  the  temperature  at  o°  were  beyond 
question.  On  several  occasions  pressure  measurements  at  o°  were  made 
at  intervals  of  30  to  60  minutes  and  no  measurable  difference  was  found. 
Similarly,  repacking  the  bulb  in  a  fresh  supply  of  ice  gave  exactly  the  same 
value. 

DEFINITION  OF  TEMPERATURE  BY  MEASUREMENT  OF  PRESSURE. 

The  procedure  in  measuring  the  pressure,  p',  was  as  follows:  First  the 
three  mercury  thermometers  on  the  manometer  were  read  to  determine  the 
temperature  of  the  mercury  column  and  scale;  then  three  to  four  settings 
of  the  barometer  were  made,  alternating  with  measurements  of  the  mano- 
meter. The  mercury  thermometers  were  read  again  at  the  close.  During 
this  interval  the  other  observer  made  as  many  readings  as  possible  of  all 
the  thermo-elements. 

Before  the  manometer  was  connected  to  the  bulb,  the  point  on  the  scale 
corresponding  to  the  reference  point  of  the  manometer1  was  determined 
once  for  all  before  the  manometer  was  connected  to  the  bulb,  by  connecting 
the  two  arms  and  raising  the  mercury  to  the  point,  as  in  a  regular  pressure 
measurement.  Subsequent  manometer  readings  were  subtracted  from  this 
fixed  level,  and  the  resulting  difference  corrected  for  the  temperature  and 
calibration  corrections  of  the  scale  and  then  reduced  to  o°.  The  barometer 
reading  was  similarly  corrected.  The  algebraic  sum  of  the  two  gave  the 
pressure  p',  in  terms  of  a  centimeter  of  mercury  at  o°  and  at  the  latitude 
and  elevation  of  the  laboratory.  Since  the  absolute  value  of  the  pressure 
does  not  enter  into  the  gas-thermometer  formula,  corrections  for  altitude 
and  latitude  are  superfluous. 

Errors  and  Corrections  inp'. — The  level  of  the  fixed  reference-point  of  the 
manometer  varies  with  the  temperature  of  the  room  because  of  the  differ- 
ence in  expansion  of  the  brass  scale  on  the  one  hand  and  of  the  glass  tube 
of  the  manometer  which  carries  the  fixed  point  on  the  other.  This  correc- 
tion can  be  calculated  from  the  expansion  coefficients  of  the  materials  and 
amounts  to  0.04  mm.  per  5°.  Its  direction  and  amount  were  checked 
experimentally  by  determining  the  fixed  point  at  two  temperatures  differing 
by  about  10°,  the  room  being  open  on  a  cold  day  for  the  one  case,  and 
closed  and  heated  for  the  other.  The  difference  found  was  0.09  mm.,  and 
that  calculated  0.08  mm. 

The  lengths  of  the  divisions  of  the  brass  scale  were  corrected  for  change 
of  temperature  by  a  formula  determined  for  this  scale  at  the  Normal 
Aichungs-Kommission,  the  absolute  length  of  the  scale  having  been  deter- 
mined at  1 6°.  In  addition,  calibration  corrections,  determined  for  each 

'Pages  19  and  27. 


58 


HIGH   TEMPERATURE   GAS   THERMOMETRY. 


millimeter  of  the  scale,  were  applied.  The  total  scale  correction  was  always 
less  than  0.15  mm.,  hence  the  temperature  measurement  by  the  adjacent 
mercury  thermometers  was  abundantly  accurate  for  this  purpose. 

The  length  of  the  mercury  column  was  reduced  to  o°  by  the  expansion 
coefficient  given  in  the  Landolt-Bornstein-Meyerh  offer  Tabellen.  This 
correction  varied  from  o  to  about  3  .  oo  mm.  As  the  mercury  thermometers 
were  calibrated  and  read  to  0.1°  the  uncertainty  in  this  correction  due  to 
uncertainty  in  the  room  temperature  may  amount  to  0.05  mm.  For  their 
calibration  the  mercury  thermometers  were  compared  with  a  Richter 
standard  thermometer  calibrated  at  the  Reichsanstalt. 

The  barometer  reading  was  corrected  to  o°  by  the  Landolt-Bornstein- 
Meyerhoffer  table  for  barometer  with  brass  scale.  Two  Fuess  barometers 
were  used.  Both  had  been  tested  by  the  Bureau  of  Standards;  one  had 
an  absolute  correction  of  0.06  mm.,  the  other  was  exact.  This  was  checked 
by  direct  comparison  of  the  two.  The  variable  error  in  the  barometer  is 
probably  about  the  same  as  in  the  manometer  reading  (0.05  mm.).  On  a 
very  windy  day,  or  during  the  approach  of  a  storm,  the  barometer  was  too 
unsteady  to  permit  satisfactory  measurements  to  be  made. 

A  further  small  correction  to  the  barometer  was  necessary  to  allow  for 
the  weight  of  the  air  column  between  the  cup  of  the  barometer  and  the  top 
of  the  mercury  in  the  open  manometer  column.  This  correction  was 
appreciable,  amounting  to  0.16  mm.  in  the  extreme  case. 

To  give  some  idea  of  the  effect  of  these  small  corrections  upon  the  final 
temperature  measurement,  it  may  be  added  that  i.oo  mm.  corresponds 
approximately  to  i°. 

To  determine  the  corrected  pressure,  p,  from  the  measured  pressure,  p', 
(see  page  53),  the  volume  of  the  unheated  space,  vlt  connecting  the  bulb 
with  the  manometer,  must  be  known.1 

TABLE  VII.—  UNHEATED  SPACE. 


Maximum 

Space. 

Volume 
before 
Apr.,  1909. 

Volume   1    Unetr- 
after      1  tainty  of 
Apr.  ,1909     volume. 

'the  errors. 

cc. 

CC. 

0 

Pt-Rh  capillary,  bulb  to  topfurnace(»/) 
Pt-Rh  capillary,  top  to  outside  furnace 

0.055 

0.055 

O.OO2 

loo      i    0.04 

•     .- 

(i;,  ")  

O.O86  l     0.086  i     O.OO3 

5O         ;      O.2O 

Pt-Rh  capillary  to  gold  capillary) 

O.  102         0.054 

1 

Gold  capillary  i     ,„ 

O.O94         O.O66 

Pt  capillary  and  Ni  valve  f  ' 

O.O25         O.O25 

Space  above  meniscus  J 

O.023         O.023 

Total  

0.385         0.309     0.45 

This  was  recalculated  because  the  dimensions  of  the  capillary  of  the 
second  bulb  were  somewhat  larger  than  those  of  the  first.  The  figures 
are  given  in  Table  VII.  This  volume  was  reduced  in  April,  1909,  by  bring- 
ing the  manometer  closer  to  the  furnace,  since  the  water-jacket  of  the 
furnace  cut  off  the  heat  so  completely  that  there  was  no  risk  in  bringing 


'See  discussion  of  this  correction,  p. 


DETAILS,  ERRORS,  AND   CORRECTIONS.  59 

the  manometer  as  close  as  possible  (35  cm).    The  volume  vt  was  thereby 
reduced  from  0.39  cc.  to  0.31  cc.,  and  the  ratio  -—  from  0.00187  to  0.00150. 

VQ 

The  volume  V0,  which  enters  into  the  correction  term  (see  page  53)  was 
determined  by  weighing  the  bulb  empty,  and  filled  with  distilled  water 
at  a  known  temperature.  A  very  accurate  determination  of  this  volume 
was  not  necessary,  the  important  requirement  being  that  the  volume 
should  not  change  during  a  run.  A  check  on  change  of  volume  was  obtained 
in  the  measurement  of  the  value  of  p0.  The  volume  of  the  bulb  at  o°,  up 
to  the  base  of  the  capillary  stem,  was  found  to  be : 

c.c. 

On  13  June,  1908  (new) 205  . 74 

On  18  June,  1908  (after  1450°) 205.75 

On  20  April,  1909 205  .82 

The  volume  of  the  unheated  space,  r,,  was  arbitrarily  divided  into  three 
portions  for  the  convenient  determination  of  its  average  temperature,  /,.  The 
first  portion,  ?,,  extended  from  the  base  of  the  stem  to  the  top  of  the  upper 
brick  of  the  furnace  (see  Fig.  i) ;  the  second  portion,  i\",  included  the  capil- 
lary stem  as  far  as  the  outside  of  the  furnace;  the  third  portion,  ^'".extended 
to  the  surface  of  the  mercury  in  the  manometer  and  included  all  of  that 
portion  of  the  unheated  space  which  remained  at  room  temperature. 

The  temperatures  of  the  portions  i1,'  and  v/'  were  determined  by  placing 
a  thermo-element  at  different  points  along  the  stem  during  several  of  the 
runs.  As  this  temperature  does  not  need  to  be  known  accurately,  a  few 
measurements  gave  a  sufficient  indication  of  the  distribution  of  temperature 
in  the  portion  of  the  "unheated  space"  within  the  furnace. 

A  liberal  estimate  of  the  degree  of  uncertainty  in  the  values  of  vt  and  tl 
has  been  made  and  is  included  in  Table  VII,  together  with  the  effect  which 
such  errors  would  have  on  the  calculated  temperature,  /,  at  the  copper 
point. 

Errors  and  Corrections  in  pj . — The  same  instrumental  corrections  apply 
to  po  as  to  p' ';  but  their  proportional  magnitude  is,  of  course,  larger.  The 
values  of  the  uncertainty  in  /  due  to  these  small  errors  will  be  found  in 
Table  X,  p.  69. 

As  appears  in  Table  X,  the  largest  possibility  of  error  in  p0  conies 
from  corrections  for  the  temperature  of  the  mercury  columns.  These 
errors  always  affect  p0  and  p  in  nearly  equal  magnitude  and  become  negligi- 
ble in  their  effect  upon  p  —  p0,  but  appear  (uncompensated)  in  p0  in  the 
denominator.  See  equation  (5)  p.  53.  To  insure  the  constancy  and  accuracy 
of  the  temperature  of  the  mercury  column,  the  manometer  was  jacketed 
over  its  entire  length  with  a  pasteboard  jacket.  This  was  sealed  tight 
at  the  permanent  joints,  and  built  up  in  removable  sections  over  the  portion 
of  the  manometer  through  which  the  height  of  mercury  varied.  A  current 
of  air  was  circulated  through  this  jacket  by  a  large-capacity  suction  jet. 
At  the  same  time  an  electric  fan  kept  a  rapid  upward  current  of  air  flowing 
over  the  entire  apparatus.  The  temperature  of  the  room  was  also  kept 
as  nearly  constant  as  possible.  Under  these  conditions  the  measurement 
of  p0'  was  reproducible  within  0.02  to  0.03  mm.,  corresponding  to  about 
0.2°  at  1500°.  This  was  checked  experimentally  on  several  occasions  by 


60  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

making  readings  of  p0'  under  different  conditions  of  room  temperature  and 
barometric  pressure. 

Changes  in  the  value  of  p0  (the  ice-point)  after  heating  to  high  tempera- 
tures have  always  been  disturbing  factors  in  gas-thermometer  measurements 
and  have  introduced  uncertainties  of  a  very  intangible  kind.  This  was 
especially  true  of  the  porcelain  bulbs  formerly  used,  where  both  changes 
of  volume  and  emission  or  absorption  of  gases  by  the  walls  occurred.  The 
restoration  of  the  platinum  metals  to  favor  as  materials  for  the  gas-ther- 
mometer bulb  has  practically  eliminated  this  uncertainty.  During  the 
present  work  small  changes  in  the  value  of  p0  have  frequently  occurred 
after  heating  to  a  high  temperature,  which  seem  not  to  be  due  to  any  change 
in  volume,  for  the  determinations  of  the  volume,  F0,  given  above  (p.  59), 
show  a  total  change  after  a  year's  work  corresponding  to  less  than  o.  i  mm. 
in  p0.  In  the  early  part  of  the  work,  the  passage  through  the  bulb  wall 
of  hydrogen  or  some  other  gas  produced  by  the  reducing  action  of  wood 
fiber  in  an  asbestos  board  insulator  within  the  furnace,  was  suspected  as 
being  the  cause  of  irregularity,  particularly  in  view  of  the  fact  that  Holborn 
and  Valentiner  had  difficulties  from  this  cause. 

Further,  it  was  several  times  observed  that  heating  the  furnace  and  bulb  to 
a  higher  temperature  than  they  had  reached  before  caused  a  slight  increase 
in  the  value  of  p0 — whether  due  to  some  gas  passing  in  from  the  outside 
or  coming  out  of  the  wall  of  the  bulb  is  not  known.  Air  dried  over  calcium 
chloride  was  used  outside  of  the  bulb  in  the  furnace  inclosure  throughout 
the  work,  and  no  indication  was  ever  obtained  of  the  passage  of  either 
oxygen  or  nitrogen  through  the  wall  of  the  bulb,  since  measurements  at 
a  given  temperature  (after  the  first  heating  to  that  temperature)  gave  the 
same  value  of  p0  within  the  error  of  measurement. 

On  one  occasion  an  almost  inappreciable  leak  in  the  manometer  connec- 
tion caused  some  uncertainty.  All  measurements  affected  by  this  error, 
when  it  was  discovered,  were  rejected. 

THE  GAS. 

Since  the  gas-thermometer  apparatus  as  arranged  for  high-temperature 
measurements  is  not  suited  to  a  determination  of  the  value  of  a  (the  pressure 
coefficient  of  the  gas  from  o°  to  100°)  with  an  accuracy  comparable  to  that 
attained  by  Chappuis,1  the  value  of  a  was  treated  as  a  constant.  The 
figures  used  were : 

For  p0  =  345  —  347mm.,  0=3665. 8  xio~6 
For  ^0  =  217  — 221  mm.,  a=3664.oxio~6 

It  will  be  recalled  that  a  number  of  independent  determinations  of  a 
for  different  pressures  were  made  (p.  40)  with  the  platin-iridium  bulb, 
but  they  show  no  appreciable  difference  from  those  by  Chappuis  within 
the  experimental  error  of  the  apparatus.  The  probable  error  in  Chappuis 's 
results  is  not  great  enough  to  affect  the  high-temperature  values. 

Pure  nitrogen  was  used  throughout  as  the  thermometric  gas.2     The 

'Trav.  Mem.  Bur.  Int.,  6  and  12,  1888  and  1902. 

2It  was  prepared  by  dropping  a  solution  of  200  grams  of  sodium  nitrite  dissolved  in  250  grams  of  water 
into  a  warm  solution  containing  350  grams  of  ammonium  sulphate  and  200  of  potassium  chromate  in  600 
of  water.  It  was  passed  through  a  mixture  of  potassium  bichromate  and  sulphuric  acid  and  stored  over 
water.  For  use  in  the  gas  thermometer  it  was  purified  by  passing  through  calcium  chloride,  hot  copper 
gauze,  potassium  bichromate  in  sulphuric  acid,  2  bottles  potassium  pyrogallate  solution,  sulphuric  acid, 
calcium  chloride,  and  phosphorus  pentoxide. 


DETAILS,  ERRORS,  AND    CORRECTION'S.  6 1 

storage  tank  was  refilled  several  times  so  that  not  all  the  gas  was  from  the 
same  original  supply;  the  filling  of  the  bulb  was  also  changed  several 
times.  The  bulb  was  first  completely  evacuated  and  heated  to  a  high 
temperature,  after  which  the  connections  and  bulb  were  rinsed  out  several 
times  with  the  purified  gas  before  the  final  filling. 

EXPANSION  COEFFICIENT  OF  THE  BULB  03). 

The  substitution  of  a  new  alloy  in  place  of  the  platin-iridium  made 
necessary  a  new  determination  of  the  expansion  coefficient  of  the  bulb 
material.  The  method  of  its  determination  and  the  comparator  used  for 
the  purpose  have  been  fully  described  in  the  earlier  pages  of  this  paper 
(p.  27)  and  do  not  require  to  be  repeated  here. 

Three  additional  precautions  were  taken  in  carrying  out  the  measure- 
ments. The  bar  was  increased  in  length  to  500  mm.  and  in  diameter  to 
6  mm.,  in  order  to  increase  the  sensitiveness  of  the  determination  and  the 
uniformity  of  temperature  along  the  bar  respectively.  In  this  case  the 
bar  was  also  made  at  the  same  time  and  from  the  same  alloy  as  the  bulb 
itself,  and  was  therefore  identical  with  it  in  composition.1 

In  ruling  the  bar,  the  lines  were  spaced  0.2  mm.  apart  instead  of  0.5  mm., 
as  in  the  previous  investigation.  This  enabled  a  greater  number  of  obser- 
vations to  be  made  within  a  narrow  region  than  before,  and  thus  made 
it  possible  to  avoid  the  error  from  parallax  described  on  page  34. 

The  third  precaution  involved  a  slight  change  in  the  comparator  itself, 
and  was  made  at  the  suggestion  of  Chappuis.  Our  custom  had  been  to 
verify  the  distance  between  the  fixed  hairs  of  the  microscopes  before  and 
after  each  heating  by  measuring  this  distance  in  terms  of  a  standard  brass 
bar  calibrated  at  the  Bureau  of  Standards.  The  brass  bar  was  then  replaced 
by  the  platin-iridium  bar  before  the  heating  began,  and  the  length  of  the 
latter  was  measured  in  terms  of  the  initial  distance  between  the  fixed  hairs, 
at  intervals  of  50°  or  100°  up  to  1000°.  This  mode  of  procedure  involved 
the  assumption  that  the  agreement  of  the  measurements  made  before  and 
after  heating  afforded  adequate  proof  that  no  change  had  taken  place  during 
heating.  The  justification  for  this  assumption  lay  in  the  facts  that — 

(1)  The  furnace  was  completely  water- jacketed  to  prevent  any  heat 
reaching  the  microscopes  from  the  furnace. 

(2)  Suitable  insulating  material  introduced  between  the  observer  and  the 
microscopes  cut  off  any  disturbing  influence  from  the  near  approach  of  the 
observer's  body. 

(3)  The  microscopes  themselves,  and  the  carriages  upon  which  they  were 
mounted,  were  connected  by  carefully  selected  invar  bars  of  negligible 
expansion  coefficient. 

(4)  The  faithful  agreement  of  all  the  measurements  on  the  standard 
brass  bar  before  and  after  the  many  heatings  left  no  reason  for  suspecting 
such  a  variation  in  the  cross-hair  distance. 

Notwithstanding  these  conditions,  it  appeared  to  Chappuis  that  some 
positive  proof  should  be  offered  that  the  distance  between  the  cross-hairs 
remained  unchanged  while  the  heating  was  going  on,  inasmuch  as  all  the 
measurements  were  made  in  terms  of  this  distance.  Accordingly,  at  his 


•The  new  bulb,  as  well  as  the  bar,  were  made  with  the  utmost  care  by  Dr.  Heraeus,  of  Hanau,  Germany, 
fjr   this  investigation. 


62 


HIGH   TEMPERATURE    GAS   THERMOMETRY. 


suggestion,  it  was  arranged  to  retain  a  standard  unheated  bar  in  the  field 
of  the  microscopes  throughout  the  readings,  so  that  the  distance  between 
the  cross-hairs  would  be  subject  to  check  at  any  time  during  the  observa- 
tions. The  arrangement  made  for  the  purpose  is  very  simple  and  effective, 
as  can  be  seen  from  the  neighboring  diagram  (Fig.  1 1 ) .  The  last  two  series 
of  measurements  were  made  with  this  appliance,  and  the  fixed  distance 
was  found  to  remain  constant  throughout  the  series  to  within  0.003  nun., 
although  on  first  setting  up  the  apparatus  a  gradual  adjustment  of  strain, 
amounting  to  0.012  mm.,  took  place  during  the  first  two  days. 


FIG.  1 1.  A  transverse  section  of  the  expansion-coefficient  furnace  at  one  of  the  openings, 
showing  the  method  of  illumination  of  the  heated  bar  and  the  standard  cold  bar  (I)  together 
with  an  arrangement  for  checking  the  distance  apart  of  the  cross-hairs  at  each  temperature. 
With  a  screen  inserted  at  a,  only  the  hot  bar  is  visible;  with  the  screen  at  b,  only  the  cold 
bar.  Compare  with  Fig.  6,  p.  29. 

The  determination  of  /3  is  subject  to  two  errors :  The  first  is  uncertainty 
of  temperature,  the  second  occurs  in  the  measurement  of  the  change  in 
length.  It  was  impossible  to  wind  the  furnace  (70  cm.  long  and  1.5  cm. 
inside  diameter,  with  two  side  openings)  so  as  to  give  a  perfectly  uniform 
temperature  along  the  bar;  but  as  the  furnace  winding  and  consequent 
distribution  of  temperature  were  varied  considerably  for  each  run,  the 
uncertainty  from  this  cause  was  eliminated  in  the  average  of  all  the  obser- 
'vations.  The  error  in  the  temperature  measurement  itself  was  probably 
not  over  2°,  which  would  give  an  error  of  less  than  0.2  per  cent  at  the  highest 
temperature.  Two  thermo-elements  with  a  common  junction  were  used, 
one  entering  from  each  end  of  the  furnace.  This  not  only  gave  a  second 
temperature  reading  in  confirmation  of  the  first,  but  a  positive  check  upon 
the  appearance  of  contamination  in  the  thermo-elements.1 

With  a  half-meter  bar  and  a  temperature  interval  extending  from  zero 
to  1400°,  the  total  expansion  amounts  to  about  7.8  mm.  The  micrometers 
reading  the  expansion  were  read  with  an  accuracy  of  0.002  mm. 

There  was  again  some  indication  of  a  small  hysteresis  in  the  expansion 
and  contraction.  Although  the  amount  was  not  much  greater  than  the 


lSee  p.  23. 


DETAILS,  ERRORS,  AND   CORRECTIONS.  63 

experimental  error,  the  measurements  indicate  that  the  bar  was  slightly 
shorter  after  each  heating  than  before,  and  that  it  gradually  regained  its 
original  length.  (See  also  p.  35.) 

The  measurements  at  room  temperature  are  given  in  Table  VIII.  The 
five  measurements  in  this  table  which  were  made  within  a  few  hours  after 
the  bar  had  cooled  from  a  high  temperature,  excluding  the  two  where  the 

TABLE  VIII.—  LENGTH  OF  PLATINUM-RHODIUM  BAR. 


Maximum 

Maximum 

Date. 

preceding 

Length  at  o". 

Date. 

preceding 

Length  at  o". 

temperature 

temperature. 

i  July     .908 

(New) 

500.068 

26  Sept.  1908 

1150° 

500.094 

6  July    1908 

900° 

500.  i  10' 

i  Oct.    1908 

2? 

5OO.  I  19 

9  July    1908 
13  July    1908 

28 
900 

500.  105 
500.098' 

6  Oct.      1908 
27  Oct.    1908 

130O 
900 

500.034- 

500.  1  08' 

17  Sept.  1908 

900 

500  .  i  08  ' 

30  Oct.    1908 

1400 

500  .  096 

19  Sept.  1908 

1  200 

500  .  090 

6  Oct.    1009 

28 

500.103 

20  Sept.  1908 

23 

500.105 

12  Oct.      1909 

22 

500.  loS1   | 

22  Sept.  1908 

1200 

500.087 

13  Oct.    1909 

IOOO 

500.  109     i 

24  Sept.  1908 

24 

500  .  096 

15  Oct.    1909 

14OO 

500.074''    : 

! 

bar  was  bent,  average  500.095 ;  while  the  ten  measurements  (excluding  the 
first)  which  were  made  two  days  or  more  after  heating  average  500.106. 
The  difference  is  only  0.002  per  cent  of  the  total  length,  or  0.12  per  cent 
of  the  total  expansion  to  1500°  or  about  0.7  per  cent  of  the  expansion  to 
300°.  This  effect  is,  therefore,  probably  responsible  for  the  observed  irreg- 
ularities between  o°  and  300°,  at  which  point  most  of  the  temperature 
measurements  were  begun.3 

THE  THERMO-ELECTRIC  MEASUREMENTS. 

The  electromotive  forces  of  the  elements  attached  to  the  bulb  were 
measured  with  a  Wolff  potentiometer.  The  standard  of  electromotive  force 
used  was  the  international  volt,  in  terms  of  which  the  E.  M.  F.  of  the  Clark 
cell  is  1.4328  at  15°,  and  of  the  saturated  cadmium  cell  used  is  1.01835 
volts  at  25°. 

Several  small  corrections  are  necessary  in  order  to  obtain  the  true  E.  M.  F. 
of  the  thermo-element.  The  calibration  corrections  of  the  potentiometer 
(Reichsanstalt  calibration)  were  all  negligible  except  that  for  the  fixed  resist- 
ance to  which  the  standard  cell  was  attached.  This  correction  amounted 
to  1.3  microvolts  in  10,000.  The  correction  for  the  change  of  resistance 
with  temperature  of  the  potentiometer  was  also  negligible.  The  E.  M.  F. 
of  the  standard  cell  varies  with  the  temperature;  hence,  the  temperature 
of  the  cell  was  read  at  each  measurement  and  a  small  correction  applied. 
The  readings  were  correct  at  21.5°.  For  a  variation  of  5°  from  this  tem- 
perature the  correction  was  2.2  microvolts  in  10,000  microvolts.  The 
resistance  of  the  contacts  of  the  potentiometer,  and  the  small  E.  M.  F.'s 
existing  at  contact  points  in  the  circuit  of  the  thermo-element,  introduced 

'After  interval  of  4  to  7  days. 

2After  heating  beyond  the  last  temperature  at  which  measurements  were  made  it  was  discovered  that  the 
bar  had  become  bent  by  sagging  under  its  own  weight. 

:'Kammerlingh-Onnes  (Konink.  Ak.  Wet.  Amsterdam,  Proc.,  10,  342,  1907)  has  found  the  same  effect 
after  cooling  platinum  to  very  low  temperatures. 


64  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

another  small  error  which  was  determined  by  placing  the  thermo-element 
in  ice  and  reading  the  E.  M.  F.  This  correction  varied  for  the  different 
elements  from  —  i  to  +4  microvolts. 

As  a  check  upon  the  absolute  value,  a  Weston  standard  cadmium  cell 
(calibration  by  the  Bureau  of  Standards)  whose  E.  M.  F.  was  read  directly 
on  the  potentiometer,  was  compared  with  the  saturated  cell  each  day.  The 
agreement  of  the  corrected  values  was  usually  within  0.5  microvolt.  As 
in  the  case  of  the  pressure  measurement,  the  absolute  value  of  the  E.  M.  F. 
is  not  of  importance,  since  it  is  used  only  for  transference  from  the  fixed 
points  to  the  gas  thermometer;  the  above  corrections  were  applied,  how- 
ever, to  reduce  the  readings  to  a  common  standard. 

The  effect  of  contamination  (p.  23)  of  the  thermo-element  wires  in 
furnace  readings  was  much  greater  than  the  above-mentioned  errors.1  Up 
to  1 1 00°  the  contamination  was  not  serious,  but  above  that  temperature 
the  wires  take  up  iridium  together  with  some  rhodium.  It  was  hoped  that 
the  replacement  of  iridium  in  the  bulb  by  rhodium,  which  is  much  less 
volatile,  would  do  away  with  this  error,  but  there  appeared  still  to  be  a 
very  small  percentage  of  iridium  or  other  contaminating  substance  in  the 
furnace  or  in  the  furnace  wire,  enough  to  affect  the  thermo-element  wires 
appreciably,  even  though  this  furnace  wire  had  been  especially  purified 
for  this  purpose. 

Although  the  task  became  much  longer  and  more  laborious,  it  was  thought 
wise  to  avoid  the  error  from  contamination,  even  of  this  diminished  magni- 
tude, rather  than  to  attempt  to  compromise  with  it  by  any  scheme  of 
approximate  evaluation.  Accordingly,  after  every  exposure  of  sufficient 
length  to  endanger  the  thermo-electric  readings,  all  the  thermo-elements 
were  removed  from  the  furnace  and  their  wires  tested  for  homogeneity. 
Where  contamination  was  found,  the  contaminated  portion  of  the  wire  was 
at  once  cut  off.  This  is  the  only  absolutely  safe  method  of  avoiding  errors 
from  this  cause,  for  it  amounts  to  the  use  of  new  thermo-elements  exclusively 
in  all  the  determinations  of  temperature  distribution  within  the  furnace  as 
well  as  for  establishing  the  absolute  temperature  of  the  metal  melting-points. 

Testing  the  Thermo-element  Wires. — A  very  simple  method  of  testing  the 
wires  for  contamination  has  been  developed,  which  consists  in  connect- 
ing the  junction  end  of  the  wire  to  be  tested,  together  with  an  uncontam- 
inated  wire,  to  the  potentiometer  and  moving  the  free  end  of  the  standard 
wire  along  the  wire  to  be  tested,  while  heating  the  contact  point  of  the  two 
with  a  blast  lamp.2  The  variation  of  the  E.  M.  F.  produced  at  this  junction 
indicates  the  degree  of  contamination  of  the  wire;  in  the  uncontaminated 
portion  this  E.  M.  F.  is  small  and  constant  within  3  microvolts.  The 
temperature  obtained  by  the  blast-lamp  flame  is  sufficiently  constant  for 
the  purpose  and  lies  between  1460°  and  1500°. 

The  wires  could  be  relied  upon  to  give  a  constant  E.  M.  F.  within  3 
microvolts  at  1000°  over  a  length  of  at  least  50  cm,  so  that  redeterminations 
of  the  fixed  points  were  not  necessary  after  cutting  off  each  small  portion 
of  contaminated  wire.  Each  test  for  contamination  was  continued  over  the 
50  cm.  of  wire  adjacent  to  the  hot  junction  and  so  served  as  a  test  for  the 

"For  a  more  thorough  discussion  of  this  effect,  see  W.  P.  White,  Phys.  Rev.,  23,  449-474,  1906. 
5W.  P.  White,  loc.  cit.,  p.  454- 


DETAILS,  ERRORS,  AND   CORRECTIONS.  65 

homogeneity  of  the  new  wire  which  replaced  the  portion  cut  off.  In  two 
cases  a  sudden  change  of  E.  M.  F.  along  the  unused  wire  amounting  to 
about  10  microvolts  showed  the  probable  presence  of  a  junction  point  in 
the  original  sample  from  which  the  wire  was  drawn.  Such  a  junction  point 
was  of  course  not  introduced  into  the  heated  portion  of  the  furnace. 

In  this  connection,  it  should  be  pointed  out  that  the  relative  weight  to 
be  given  to  the  element  inside  the  bulb,  as  compared  with  the  outside 
elements,  is  greater  at  temperatures  above  1100°  than  at  temperatures 
below,  for  two  reasons: 

(1)  The  temperature  at  the  middle  of  the  bulb  is  not  so  much  influenced 
above  1100°  by  the  temperature  of  the  lower  part  of  the  furnace  as  it  is 
below  1 1 00°. 

(2)  The  outside  elements  are  much  more  subject  to  contamination  than 
the  inside  element  by  reason  of  the  protection  afforded  the  inside  element 
by  the  intervening  bulb  walls  against  contaminating  material  from  the 
heating  coils  or  the  furnace. 

This  is  well  shown  by  the  data  in  Table  XV,  p.  105,  on  the  melting-points 
of  diopside,  nickel,  and  cobalt.  In  the  first  measurements  of  these  temper- 
atures, the  elements  were  left  on  the  bulb  through  several  runs,  in  con- 
sequence of  which  the  readings  of  the  outside  elements  on  the  bulb  steadily 
decreased,  whereas  the  temperatures  derived  from  the  inside  elements  are 
fairly  constant.  The  contamination  of  the  inside  element  was  found  to  be 
less  in  amount  and  distributed  over  a  region  of  more  constant  temperature. 

For  insulating  the  thermo-element  wires  from  the  bulb  and  furnace, 
capillary  tubes,  both  of  Marquardt  porcelain  and  of  silica  glass,  were 
employed.  The  Marquardt  tubes  are  open  to  the  objection  that  they  are 
very  porous  and  offer  little  protection  against  contamination.  The  silica 
glass  capillaries  protected  the  wires  very  much  better,  but  at  1100°  and 
above  they  devitrified  rapidly  and  at  the  end  of  a  measurement  at  1400° 
or  over  fell  from  the  wires  in  small  fragments,  so  that  the  wires  had  to  be 
taken  off  and  reinsulated  after  a  single  run. 

For  the  convenience  of  others  who  may  confront  similar  problems,  it 
may  be  added  that  such  extreme  precautions  as  cutting  off  the  elements 
at  the  first  sign  of  contamination  are  excessive  for  most  purposes.  The 
region  of  highest  temperature,  and  therefore  of  most  rapid  contamination 
in  a  good  furnace,  is  also  a  region  of  constant  temperature.  Contamina- 
tion would  therefore  produce  little  effect  upon  the  reading  of  the  thermo- 
element until  it  had  crept  out  into  the  colder  parts  of  the  furnace,  which 
it  will  do  slowly  during  long  exposures.  The  distribution  of  the  contami- 
nation in  an  aggravated  case  is  shown  in  the  table  on  page  66,  which  is 
arranged  in  such  a  way  that  not  only  the  magnitude  of  the  contamination 
but  also  its  distribution  with  respect  to  the  bulb  is  roughly  shown.  The 
electromotive  forces  are  determined,  as  has  been  explained,  by  bringing 
successive  points  of  the  contaminated  wire  into  contact  with  an  uncon- 
taminated  one  in  a  blast  flame  (temperature,  1460°- 15 00°),  the  cold  junction 
being  maintained  constant  at  o°.  The  absolute  magnitude  of  the  numbers 
in  the  column  "  before  exposure"  represents  the  electromotive  force  between 
two  uncontaminated  platinum  wires  of  (nominally)  equal  purity.  Its 
constant  value  is  a  measure  of  the  homogeneity  of  the  new  wire.  Its 


66 


HIGH   TEMPERATURE    GAS   THERMOMETRY. 


departure  from  this  constant  value  "after  exposure"  is  a  measure  of  the 
contamination  it  has  received.  Slight  irregularities  are  the  result  of 
variations  in  the  blast-flame  temperature.  Such  observations  merely 
serve  to  furnish  information  about  the  distribution  and  approximate 
amount  of  contamination  received  by  the  element,  but  do  not  of  themselves 
provide  the  data  to  correct  its  reading  in  a  particular  case. 

It  was  found  desirable  to  glow  the  thermo-elements  occasionally  by 
sending  through  them  a  current  of  from  12  to  17  amperes,  depending  upon 
the  size  of  the  wire.  The  glowing  served  to  clean,  soften,  and  straighten 
the  wires,  but  did  not  affect  the  permanent  electro-motive  force  of  the 
element  if  not  too  long  continued.  The  electro-motive  force  of  a  new 
element,  however,  was  found  to  change  on  heating  by  as  much  as  10  micro- 
volts in  10,000.  Elements  made  from  freshly  drawn  wire  were  therefore 
always  glowed  for  5  minutes  before  they  were  calibrated  and  used. 


1     Centi- 

Before 

After 

meters 

expos- 

exposure 

from  hot 

/._• 

(micro- 

junction. 

(micro- 
volts). 

volts). 

40 
35 

-I 

-t 

Outside  of  furnace.  . 

....       30 

-8 

~  7 

25 

-9 

-  6 

Bend  of  stem  

.  .  .  .  '      2O 

-6 

—  10 

«5 

-5 

—   3 

12 

-5 

+    2 

1O 

—  5 

+  9 

Shoulder  of  bulb  .  .  . 

....;     8 

-5 

+83 

6 

-5 

+83 

4 

-6 

+4' 

2 

-6 

+  55 

Middle  of  bulb  

.  .  .  .:         O 

-8 

i 

Integration  of  Temperatures  over  the  Bulb. — By  the  method  which  has 
been  already  described  (p.  54),  the  differences  of  temperature  between  the 
ends  of  the  bulb  and  the  middle  were  determined  differentially  by  means 
of  platinum  wires  attached  to  the  bulb  itself.  Temperatures  about  the 
circumference  were  measured  by  separate  thermo-elements,  as  it  was  not 
practicable  to  measure  these  differences  differentially  because  of  the 
necessity  of  passing  a  platinum  binding  wire  around  the  bulb  to  hold  the 
four  elements  in  position.  A  check  on  the  accuracy  of  this  differential 
method  was  obtained  by  using  in  one  case  a  thermo-element  at  the  top 
shoulder  of  the  bulb  and  thus  measuring  the  temperature  at  this  point 
both  directly  and  differentially  by  means  of  the  platinum  wire  of  this 
element.  The  two  temperatures  agreed  within  0.8°  when  the  deviation  from 
the  middle  was  6°;  when  the  temperatures  at  the  middle  and  top  were 
nearly  equal,  the  two  methods  agreed  to  0.1°. 

Table  IX  contains  approximate  values  of  — — ,  the  rate  of  change  of 

E.  M.  F.  with  temperature,  at  various  temperatures  from  400°  to  1500°,  both 
for  the  10  per  cent  rhodium  alloy  and  for  the  20  per  cent  alloy  of  which 
the  bulb  was  made.  The  data  for  the  20  per  cent  alloy  (which  need  be 
only  approximate)  were  obtained  by  two  methods : 


DETAILS,  ERRORS,  AND    CORRECTIONS.  67 

(1)  An  element  was  made  up  by  combining  a  platinum  wire  with  the 
20  per  cent  rhodium  bar  used  for  the  expansion  coefficient  determination, 
and  its  readings  were  compared  directly  with  those  of  a  10  per  cent  rhodium 
element  in  the  melting-point  furnace. 

(2)  A  platinum  wire  was  connected  from  the  stem  of  the  gas-thermometer 
bulb  outside  of  the  furnace  to  the  ice  box,  and  the  E.  M.  F.  determined 
against  the  standard  platinum  wire  attached  to  the  middle  of  the  bulb. 
In  both  cases  the  E.  M.  F.  of  the  junction  of  platinum  with  the  rhodium 
alloy  at  room  temperature  was  applied  as  a  correction. 

TABLE  IX. — VALUES  OK  ^  FOR  THE  ALLOYS  90  PT  10  RH  AND  80  PT  20  RH. 

OoPtioRh      8oPt2oRh  j 
Temperature,    (microvolts     (microvolts  ; 
per  i°).  per  i°) 


400 

95 

II.  5 

600 

10.2 

12.8 

800 

10.8 

14.2 

IOOO 

11.5 

15.6 

1200 

11.9 

16.9 

I4OO 

12.  1 

17.5 

I5OO 

12.  1 

17.8 

In  order  to  obtain  the  true  E.  M.  F.  corresponding  to  the  temperature 
as  measured  by  the  pressure  of  the  gas  in  the  bulb,  it  is  necessary  to  integrate 
the  various  readings  over  the  surface  of  the  bulb.  The  following  arbitrary 
weights  were  given  to  the  different  positions  of  elements  on  the  surface : 

Top  axis  (position  i ) 5 

Top  shoulder  (position  2) 20 

Middle  (4  elements)  (position  4) 55 

Bottom  shoulder  (position  6) 15 

Bottom  axis  (position  7) 5 

The  elements  on  the  axis  at  both  top  and  bottom,  although  sometimes 
deviating  from  the  others,  have  comparatively  small  weight,  as  they  affect 
only  a  small  portion  of  the  total  volume.  The  element  at  the  lower  shoulder 
of  the  bulb  is  given  less  weight  than  that  at  the  top  because  of  the  smaller 
volume  of  the  lower  half,  due  to  the  presence  of  the  reentrant  tube. 

It  was  easy  to  show  experimentally  that  it  matters  very  little  what  these 
relative  weights  assigned  to  the  different  readings  may  be,  since  the  total 
correction  was  always  small.  In  a  number  of  cases,  two  different  settings 
of  the  temperature  distribution  were  made  at  each  temperature,  one  in 
which  the  elements  at  the  top  and  bottom  shoulders  of  the  bulb  were  made 
equal  to  the  middle,  and  one  in  which  the  elements  at  top  and  bottom  on 
the  axis  of  the  cylinder  were  made  equal  to  the  middle.  The  pressures 
corresponding  to  these  two  settings,  reduced  to  the  same  reading  of  the 
standard  element,  are  shown  for  several  typical  cases  in  the  table  below. 

i  ,f.  I  Pressure  when  Pressure  when  : 

Date.  lemper-     Ij4and7were   2,  4  and  6  were' 

equal.  equal. 

mm.  mm. 

22  January,  1909 1082     ,     1038.82  1038.64 

2  July,  1909 1395     :     1285.43  1285.17 

17  September,  1909...      1489    ;     1331.40  1330.63 


68 


HIGH   TEMPERATURE    GAS   THERMOMETRY. 


DETAILS,  ERRORS,  AND  CORRECTIONS. 


69 


It  is  evident  that  even  without  any  correction  for  the  different  distribu- 
tion in  the  two  cases,  the  readings  agreed  within  0.2  to  0.8  mm.  or  about 
0.2  to  0.9°,  so  that  the  variation  between  any  two  arbitrary  sets  of  weights 
which  might  be  given  to  the  different  readings  must  lie  well  within  this 
limit. 

The  thermo-element  connections,  from  the  gas-thermometer  bulb  to  the 
potentiometer,  are  shown  diagrammatically  in  Fig.  12. 

SUMMARY  OF  THE  ERRORS. 

The  effect  on  the  final  temperature  of  all  the  errors  and  corrections  which 
have  been  discussed  in  this  section  is  shown  in  summarized  form  in  Table 
X.  The  figures  of  this  table  serve  to  emphasize  the  statements  already 

TABLE  X. — ESTIMATED  KRRORS  AND  THEIR  EFFECT  ON  THE  VALUE  OF  /. 


Quantity  affectec 


Source  of  error 


Amount  of  error.  Ivffect  on  .'. 

At  400°.  At  1500".         At  400°.    ;     At  1500° 


(A)  Temperature   Temperature  differences 


of  gas. 

over  bulb  surface  
Variability  

2 

o 

mv. 

5 
i 

mv. 
mv. 

±0 
0 

2° 

db 

o 
o 

4° 
I  ° 

(B)    p0  

Reference  point  

o 

.  02  mm  . 

i  ° 

.  02  mm  . 

±0 

04° 

•j 

o 

15° 

P0  

Manometer  setting  

O 

.02  mm. 

o 

.02  mm. 

±0 

04° 

db 

o 

15° 

Po  

Scale  corrections  

O 

01  mm. 

o 

.01  mm. 

±0 

02° 

=t 

o 

07° 

Po  

Temp,  of  mercury  

0 

03  mm. 

o 

.03  mm. 

±0 

06° 

db 

o 

23° 

Po  

Barometer  setting  

o 

03  mm. 

o 

03  mm. 

±0 

06° 

=t 

o 

23° 

Po  

Temp,  of  barometer.  .  .  . 

o 

03  mm. 

o 

03  mm. 

±O.O6°      d= 

0.23° 

Po  

Variations  in  p0  

o 

o-.  05  mm 

0 

o  to 

±0.3° 

p  

Reference  point  

0 

02  mm. 

0 

02  mm. 

=*=  o  .  02  ° 

o 

p  

Manometer  setting  

0 

02  mm. 

0 

02  mm. 

±0 

02 

o 

p  

Scale  corrections  

o 

02  mm. 

o 

02  mm. 

±o 

02° 

o 

p  

Temp,  of  mercury  

o 

07  mm. 

o.2omm. 

±O.O70       d= 

o. 

05° 

p  

Barometer  setting  

o 

03  mm. 

0 

03  mm. 

±o 

03° 

i 

o. 

OI  ° 

p  

Barometer  temp  

o 

03  mm. 

o 

03  mm. 

±0 

03° 

=±= 

o. 

01° 

p  

Unheated  space<  ,  !  '  ' 

o 
o 

020  CC. 

5-50° 

O.O2OCC. 

o.  5°-ioo° 

=*=o 

±0 

07° 

01° 

=fc 

o. 

0. 

5° 

/?.  . 

Temperature  

, 

0° 

2 

0° 

±0 

02° 

dt 

o. 

1  1° 

0  

Expansion  

o 

oojmm. 

o 

ooSmm. 

±0 

02° 

-.i. 

o. 

09° 

/3  

Hysteresis  in  expansion  . 

o 

01  mm. 

o 

01  mm. 

±0 

04° 

=*= 

0. 

10° 

(C)  E.  M.  F.... 

Instrumental  correct'ns 

I 

mv. 

2  mv. 

±0 

1° 

sfc 

o. 

2° 

E.  M.  F.... 

Contamination  

o 

0-12  mv. 

o 

o  to 

+  1.0° 

E.  M.  F... 

Integration  over  bulb  .  . 

3 

mv. 

12  mv. 

±0 

J° 

=*= 

'• 

o° 

(D)  Fixed  points 

Instrumental  correct'ns 

, 

mv. 

2 

mv. 

±0 

I  ° 

db 

o. 

2° 

Fixed  points 

Contamination  

0 

O- 

-10  mv. 

0 

O 

0 

—  1  .0° 

Fixed  points 

Variation  in  given  charge 

Specific,  i- 

10  mv. 

Specific, 

O. 

1° 

to  1.0° 

Fixed  points 

Variation  between  dif- 

ferent charges  

Specific, 

1-20  mv. 

Specific, 

o. 

tO  2.0° 

made,  that  the  greatest  present  uncertainty  in  the  high -temperature  gas 
scale  arises  from  the  lack  of  uniformity  in  an  air  bath,  which  not  only  leads  to 
uncertainty  as  to  what  is  the  true  temperature  of  the  gas  in  the  bulb., 
but  also  to  errors  in  the  transference  by  the  thermo-element.  The  next 


70  HIGH  TEMPERATURE   GAS  THERMOMETRY. 

largest  uncertainty,  due  to  the  limitations  in  the  purity  and  reproducibility 
of  the  substances  available  for  fixed  points,  is  not  directly  chargeable  to  the 
gas  thermometer.  In  this  connection,  considerable  more  work  needs  to  be 
done  on  the  high  thermometric  points,  comparable  in  thoroughness  to  the 
work  in  low-temperature  thermometry  of  Richards,  Dickinson,  and  others, 
on  the  sodium  sulphate  transition-point. 


13.  EXPERIMENTAL  DATA  AND  CALCULATED  RESULTS. 

EXPANSION  COEFFICIENT  OF  PLATINUM-RHODIUM. 

In  Table  XI  are  given  the  experimental  data  on  the  expansion  coefficients 
of  the  alloy  80  per  cent  platinum,  20  per  cent  rhodium.  In  the  first  column 
is  given  the  date  of  the  series,  in  the  second  and  third  columns  the  readings 
of  the  thermo-elements  at  the  middle  of  the  bar,  corrected  for  zero  error 
and  the  temperature  of  the  cadmium  cell.  The  12  other  readings  taken 
with  each  element  at  each  temperature  at  different  points  along  the  bar 
can  not  be  given  here,1  but  the  fourth  and  fifth  columns  contain  the  readings 
of  the  thermo-element  corrected  to  represent  the  integrated  temperature 
along  the  bar.  For  convenience,  the  integration  was  made  in  terms  of 
microvolts  instead  of  degrees.  The  sixth  and  seventh  columns  contain  the 
temperatures  corresponding  to  the  readings  in  columns  4  and  5,  and  the 
eighth  column  contains  the  mean  of  these  two  temperatures.  The  micro- 
meter readings  are  not  given,  but  in  column  9  will  be  found  the  expansions 
reduced  to  millimeters  for  that  portion  of  the  bar  lying  between  the  o  and 
50  cm.  marks  on  the  ends.  Each  of  these  represents  the  mean  of  eight 
settings  at  each  end  of  the  bar.  In  the  last  column  are  given  the  values 
of  the  mean  expansion  coefficient  from  o°,  calculated  by  dividing  the  expan- 
sion by  the  length  at  o°  and  by  the  temperature. 

For  convenience  of  comparison,  the  values  of  /3  at  the  nearest  round 
temperatures  were  interpolated  linearly  between  the  observations  in  each 
series,  and  the  results  are  given  in  Table  XII.  Values  interpolated  between 
these  values  are  given  in  parentheses. 

The  table  shows  that  the  percentage  error  at  300°  is  greater  than  that  at 
1200°  and  above,  probably  on  account  of  the  larger  effect  of  the  hysteresis 
in  the  expansion  and  contraction,  already  discussed  on  page  63.  The 
agreement  of  the  results  is  very  satisfactory,  particularly  in  view  of  the 
fact  that  each  series  represents  an  entirely  different  curve  of  temperature 
variation  along  the  bar.  In  some  cases  the  temperatures  at  the  end  were 
lower  than  at  the  middle,  in  others  higher  than  at  the  middle,  and  in  one 
series  one  end  was  higher  and  the  other  lower.  The  mean  of  all,  therefore, 
probably  eliminates  any  error  which  might  arise  from  variation  of  tempera- 
ture along  the  bar. 

The  results  are  represented  within  the  limits  of  error  by  the  straight-line 
equation 

io6/3=  8.79+o.ooi6i/ 

'See  p.  36  for  an  example  "of  such  readings,  showing  distribution  of  temperature  in  the  case  of  the  platin- 
iridium  bar. 


EXPERIMENTAL   DATA   AND    CALCULATED   RESULTS.  JI 

This  may  be  compared  here  with  the  expansion  coefficients  between  300° 
and  1000°  determined  by  the  authors  for  the  10  per  cent  iridium  alloy,1 
and  by  Holborn  and  Day2  for  the  20  per  cent  iridium  alloy  and  for  pure 
platinum : 


90  Pt  10  Ir,          io60=8.84+o.ooi3i/ 
Pt,         io6/3=  8.  87+0.00132* 

TABLE  XI.  —  OBSERVATIONS  OF  EXPANSION  COEFFICIENT,  0. 

Thermo  elements. 

Date. 
W               Z           W  cor. 

Zcor. 

Temperature. 
By  W         By  Z         Meat 

Expansion  from  o°. 

Milli- 
.     meters  on       io"0 
500  mm. 

1908. 

0 

0 

0 

Sept.  21  2261 

2251 

2312 

2298 

301.4 

301. 

4 

301. 

4 

I. 

404 

9.32 

3  '97 

3187 

3273 

3258 

404.6 

405. 

4 

405. 

0 

I. 

912 

9-44 

4169 

4153 

4257 

4237 

506.0 

507. 

506. 

6 

2. 

434 

9.61 

5157 

5140 

5237 

5212 

603.9 

605. 

I 

604. 

5 

2. 

950 

9.76 

6197 

6178 

6286 

6262 

"05.4 

707. 

2 

706. 

3 

3.500 

9.91 

7264 

7238 

7362 

7333 

8o6.2 

807. 

8 

807. 

o 

4 

064 

10.07 

8361 

8335 

8457 

8420 

905.9 

906. 

7 

906. 

3 

4 

640 

10.24 

9509 

9470 

9599 

9552 

i  006  .  9 

1006. 

8 

1006 

8  | 

5- 

241 

10.41 

10662 

lo6l  1 

10733 

10675 

1104.5 

1  103. 

4 

1  104. 

0 

5- 

828 

10.56 

1  1963 

11896 

12018 

1  1921 

1215.3 

1210 

2 

1212 

8 

6. 

469 

10.67 

Sept.  25.  ...      1817 

1801 

1848 

1831 

248.7 

248 

4 

248.6       1. 

'54 

9.28 

2756 

2735 

279' 

2768 

353-4 

352 

9 

353 

2 

i  . 

666 

9-43 

3699 

3674 

3726 

3702 

451.8 

452 

0 

45' 

9 

2 

158 

9-55 

4686 

4655 

4691 

4662 

549-7 

550.3 

550 

o 

2.668 

9/0 

57" 

5679 

5691 

5660 

648.2 

649 

2 

648 

7 

3.191 

9.84 

6820 

6788 

6772 

6742 

751.2 

752 

6 

75' 

9 

3 

757 

9-99 

7847 

7813 

7754 

7720 

842.2 

843 

2 

842.7 

4.262 

10.  1  I 

8980 

8945 

8845 

8809 

940.8 

94  1 

4 

941 

i 

4 

827 

10.26 

10140 

IOIO2 

9939 

9901 

1036.4 

'037 

o 

1036 

8 

5 

403 

10.42 

11368 

11327 

1  1  109 

1  1063 

1136.9 

1  136 

4 

1  136 

7 

6 

OI2 

10.58 

Oct.  3  2291 

2272 

2302 

2283 

300.3 

299 

s 

300 

i 

i 

384 

9.22 

3228 

3205 

3250 

3228 

402.2 

402 

2 

402 

2 

i 

899 

9-44 

4208 

4l8l 

4243 

4215 

504.6 

504 

9 

504 

8 

2 

432 

9.63 

5205 

5'75 

5247 

5216 

604.8 

605 

5 

605.2 

2 

964 

9  79 

6238 

6206 

6281 

6249 

704.9 

705 

9 

705 

4 

3 

5" 

9-95 

7297 

7263 

7342 

7309 

804.4 

805.5 

805 

O 

4 

069 

10.11 

8401 

8365 

8446 

8408 

904.9 

905 

6 

905 

3 

4 

644 

10.26 

9536 

9497 

9576 

9534 

1004.9 

1005 

2 

1005 

.      5.231 

10.41 

10675 

10647 

10710 

10670 

I  102.  6 

1  103 

O 

1  102 

8 

5 

830 

10.57 

11884 

11857 

1  1926 

11875 

1207.5 

1206 

2 

1206 

8 

6 

466 

10.71 

Oct.  29  8419 

8377 

8366 

8324 

897.4 

898 

O 

897 

7 

4 

618 

10.29 

955' 

9507 

9436 

9392 

992.6 

992 

8 

992 

7 

5 

169 

10.41 

10706 

10663 

10539 

10496 

1088.0 

1088 

2 

1088 

i 

5 

752 

10.57 

11884 

11849 

.1786 

11751 

1195.6 

1195.7 

"95  7     6 

401 

10.70 

,m.        "w" 

13104 
D 

'3>34 
W  cor. 

13101 
D  cor. 

1309.9 

ByW 

1309.8 
By  D 

1309 

•9 

7  '54 

10.92 

Oct.  13  2304 

2301 

2235 

2232 

293.0 

293 

.0 

293 

.0 

i 

352 

9.23 

6222 

6217 

6180 

6175 

695.2 

695 

•  9 

695 

.6 

3 

.452 

9.92 

9501 

9494 

9493 

9486 

997  .6 

998 

.  i 

997 

9 

5 

.190 

10  40 

Oct.  14              9540 

9536 

9542 

9544 

1001  .9 

1003 

.  i 

1  002 

•  5 

5 

.200 

10.37 

10666 

10663 

10690 

10691 

i  101  .9 

1  102 

•  5 

1  102 

.2 

;  5 

.8)1 

10.54 

11839 

11836 

11783 

1.783 

1195.4 

"95 

-7 

"95 

.6 

6 

•410 

10.72 

12998 

12993 

13121 

13120 

1308.9 

1308 

.(> 

.308 

.8 

i  7 

.156 

10.93 

14183 

14170 

14390 

14372 

1413.4 

141  i  .6 

1412 

•  5 

;  7 

.832 

ii  .09 

'See  pp.  27-39. 
"Am.  Jour.  Sci.  (4),  11,  3 

74-390,  i 

901.     Ann 

d.  Phys.  (4),  4, 

104-122, 

901. 

72  HIGH  TEMPERATURE  GAS  THERMOMETRY. 

TABLE  XII. — VALUES  OF  io'/3  AT  ROUND  TEMPERATURES  FOR  THE  ALLOY  80  PT  20  RH. 


Temp. 

21  Sept. 
1908. 

25  Sept. 
1908. 

3  Oct. 
1908. 

29  Oct.           13  Oct.      '     14  Oct.            M 
1908.               1909.               1909 

250 
300 
350 
400 
450 
500 
550 
600 
650 
700 
750 
800 
850 
900 
950 
IOOO 

1050 

I  IOO 

1150 

120O 
125O 
I3OO 
1350 
I4OO 
1450 
I5OO 

9.3' 
(9-37) 
9-43 
(9-52) 
9.60 
(9.67) 
9-75 
(9.83) 
9.90 
(9.98) 
10.06 
(10.  14) 
10.23 
(10.31) 
10.40 

(10.47) 
10.55 
(10.60) 
10.65 

9.28 
(9.36) 
9  43 
(9-49) 

££; 

&% 

9.84 
(9-92) 

/  "^ 
(10.06) 

IO.  12 

(10.20) 
10.27 

(10.36) 

10.44 
(10.52) 
10.60 

(10.67) 

I 

9.22 

(9-33) 

9  44 
(9-53) 
9.62 

(9-7") 
9-79 
(9-86) 
9-94 
(10.02) 

10.10 

(10.17) 
10.25 
(10.32) 
10.40 
(10.48) 
10.57 
(10.64) 
10.71 

9-24 

!   (9.33) 

i     (9-4') 
i     (9-50) 
i     (9.58) 
i     (9-67) 
'     (9-76) 
;     (9.84) 
':      993 
(10.01) 

(10.09) 

!  (io.  16) 
10.29       (10.24) 
(10.36)  !  (10.32) 
10.42         10.40 
(10.50)  I  
10.59    !  
(io  65)  ; 

9.28 
9.36 
9-44 
9.52 
9.61 
9.69 
9-77 
9.84 
9.92 

10.00 

10.08 

io.  15 
10.24 
10.32 

10.39 
10.47 
10.55 
10.62 
10.69 

10.81 

10.91 

10.99 

1  1  .07 

(11.15) 
(11.23) 

10.37 
('0.45) 
10.54 
(10.63) 
10.73 
(10.82) 
10.92 
(10.99) 
1  1  .07 

10.71     I  
(io  81)  1 

i  o  .  90    !  

(10.99)  '  

...... 

i                  i 

GAS-THERMOMETER  DATA. 

In  Table  XIII  are  given  the  observed  gas- thermometer  data.1  In  the 
first  column  is  the  date  of  measurement.  The  measurements  are  numbered 
chronologically  in  the  second  column  for  convenience  of  reference.  In  the 
third  column  is  the  measured  pressure,  p'  (or  /></),  in  millimeters  of  mercury 
at  o°,  corrected  as  described  on  pages  57-58.  The  application  of  the 
correction  for  unheated  space  (see  p.  58)  gives  the  pressure  p  (or  p0)  which 
is  found  in  the  fourth  column.  In  the  fifth  column  is  the  value  of  the 
temperature,  /,  calculated  by  formula  (5)  on  page  53.  In  column  6  are 
given  the  readings  of  the  standard  thermo-elements  in  microvolts,  and  in 
column  7  the  positions  of  these  elements  on  the  bulb;  for  the  significance 
of  these  figures  see  Fig.  8  and  note  on  page  55.  In  the  last  column  are 
given  the  other  elements  which  were  used  on  the  bulb,  together  with  their 
positions  designated  in  the  same  way.  The  italicized  letters  represent  single 
platinum  wires  instead  of  thermo-elements. 

A  few  measurements  in  which  the  value  of  p0  changed  by  more  than  o.i 
per  cent  have  been  omitted ;  their  position  .is  shown  by  the  absence  of  their 
corresponding  serial  numbers. 

'For  the  measurements  in  the  table,  seven  furnaces  were  employed,  using  three  different  coils  of  platinum 
wire  of  about  400  grams  each.  One  of  these  furnaces  was  wound  on  the  outside,  the  other  six  on  the  inside 
of  the  tube.  It  was  possible  to  rewind  the  wire  at  least  once  after  the  furnace  had  burned  out.  Failure 
always  cccurred  several  centimeters  away  from  the  bulb  in  the  end  portions  of  the  furnace,  which,  in  order 
to  secure  uniformity  of  temperature  over  the  bulb,  had  to  be  considerably  superheated.  Only  one  measure- 
ment was  made  at  the  palladium  point,  as  this  one  rendered  the  furnace  unfit  for  further  use;  the  conditions 
of  this  measurement  were,  however,  perfect. 


EXPERIMENTAL   DATA   AND   CALCULATED   RESULTS. 

TABLE  XIII. — OBSERVED  GAS-THERMOMETER  DATA. 
Gas  Filling  No.  i. 


73 


Date.             i  No.     p'  (or  pa')  \   p  (or  pa) 

i               i     j          ! 

t 

Standard      !  Posi- 
elements.      j  tion. 

1 

Other  elements 
and  positions. 

1908. 

I 

30  Nov  i       217.65       217.63 

0° 

30  Nov  2      1037.77      1042.72 

1079.87 

'W'io443\  14 

Z(i),  8(9) 

X   10491;  !  8 

i  Dec  3       217.45       217.43 

0 

.... 

2  Dec  5       217.10       217.08 

o 

3  Dec  6       948.81       952.84 

960.59 

W    9061!  :  4 

Z  (i),  8(9) 

X     9100;     8 

4  Dec  7       217.12       217.10 

o 

1  6  Dec  8       217.08       217.06 

o 

t     17  Dec  9     1038.50     1043.48 

1083.61 

W  10483!     4 

Z  (i),  8(9) 

X   10555;     8 

18  Dec  10       217.  18       217.  16 

0 

19  Dec  n      1038.57     1043.56 

1083.77 

•  W  10473!     4 

Z  (i),X  (9) 

X     I0512J    :   8 

21  Dec  12       217.06       217.04 

0 

23  Dec  15       217.49       217.47 

o 

.... 

:     24  Dec  16     1242.38     1249.71 

1365.71 

A    13866!  '  4 

Y  (i),  S  (9) 

X      ..../     8 

28  Dec  17       217.57       217.55 

0 

1909- 

22  Jan  18     1039.78      1044.74 

1082.84 

A    10502!      4 

Z  (9),  B  (1.3) 

Y    10612;  i  8 

W  (2  3),  S  (6  7) 

x  (7.3) 

22  Jan  19      1038.82      1043.79 

1081.87 

A    10506!   '  4 

Do. 

Y    10584;     8 

22  Jan  20     1037.85      1042.83  ! 

1080.89 

A    I0498\  !  4 

Do. 

Y    io555/     8 

23  Jan               21       217  36       217  34 

o 

25  Jan  22       543-01        544  07  , 

418.40 

A     3414      4-5 

Do. 

Y     3436       8 

25  Jan  23       542-27       543  32  , 

417-43 

A     3408      4.5 

Do. 

Y     3435       8 

26  Jan  24       703  .78       705  .  8  1  ' 

629.80 

A     5510    i  4.5 

Do. 

Y     5550    I  8 

26  Jan  25       702.64       704.67 

628.34 

A     5501       4.5 

Do. 

Y     5529      8 

26  Jan  26      949.56       953.631 

960.22 

A     9090       4.5 

Do. 

Y     9159      8 

26  Jan               27       948  1  5       952  23 

058   41 

A     9075       4-  5 

Do. 

y?'-'  .  ^  § 

Y     9119       8 

26  Jan  28      1039.03      1044.05 

1083.01 

A    10515       4.5 

Do. 

Y    10593       8 

26  Jan  29     1037.92      1042.93 

1081.56 

A    10505       4.5 

Do. 

Y    10556       8 

27Jan  30       217.33       217.31 
28  Jan  31        542.87       543.92 

0 

418.30 

A     3410       4.5 

Do. 

| 

Y     3436       8 

28  Jan  32       542-07       543  .11 

417.25 

A     3404      4.5 

Do. 

j                                   j 

Y     3425     ,  8 

28  Jan  33       704.06       706.07 

630.21 

A      55'4       4-5 

Do. 

Y     5553       8 

28  Jan  34       703.35       705.37 

629.31 

A      5510    i  4-5 

Do. 

Y     5537       8 

28  Jan  35       948.96       953-O5 

959.46 

A     9087       4-5 

Do. 

Y     9142;     8 

28  Jan  36       949.86       953-97 

960.69 

A     9098!     4.5 

Do. 

Y     9163!     8 

28  Jan    .            37      1038  50     1043   57 

1082  .23 

A    105  in     4.5 

Do. 

Y    I0576/     8 

74  HIGH   TEMPERATURE   GAS  THERMOMETRY. 

TABLE  XIII— OBSERVED  GAS-THERMOMETER  DATA — Continued. 
Gas  Filling  No.  i — Continued. 


Date.              H 

o.     p'(or  p0') 

i 

P(or  p0)  j 

Standard        Posi- 
elements         lion. 

Other  elements 
and  positions. 

1909. 

i 

i 

28  Jan  3 

8     1038.99 

1044.06 

1082.90     A    10512!     4.5   Z  (9),  B  (1.3),  W 
Y    io585/     8       (2.  3),  5(6.7)^(7-3) 

28  Jan  i  39     1039.61 

1044.68 

1083.68     A    I0509\     4.5 

Y    10617]     8 

Do. 

29  Jan  40  i    217.37 

217-35 

0°                       

29  Jan  41  ,    949  32  1 

953  38 

959.78     A     9086 

45 

Y     9156 

8 

Do. 

29  Jan  42       948  .  58 

952.66 

958.81     A     9085 

4-5 

Y     9131 

8 

Do. 

29  Jan  43      1039.29 

1044.34 

1083.15     A    10515 

4-5 

Y    10595 

8 

Do. 

29  Jan  44     1038.49 

1043.56 

1082.09     A    10511 

4-5 

Y    10568 

8 

Do. 

29  Jan  45      1039.63 

1044.71 

1083.58     A    10508 

4-5 

Y    10617 

8 

Do. 

jo  Jan  46       217.39 

217.37 

o                  .... 

Gas  Filling  No.  2 

1909. 

i 

18  Feb 

17       346.74 
18       346.78 

346.70 
346.74 

0 

o                 .... 

22  Feb  

A     2487]     4-5 

W(i.3),B(2.2), 

23  Feb  49       745  .09 

746.19 

319.55     D     2483 

4-5 

X  (6.2),  8(7.3),  ' 

Z     2462 

8 

Y(.2) 

A     3414 

45 

Do. 

23  Feb 

c0       866  47 

868.  15 

4  i  8  .  40     D     3406 

4    5 

7W            uwv*.^/ 

Z      3385 

s'5 

A     4451 

4-5 

Do. 

23  Feb 

51       995  97 

998  .  38 

524.71      D     4439 

4-5 

Z      4413 

A     5510 

4-5 

Do. 

23  Feb  

52      1122.39 

1125.61 

629.37     D     5495 

i  4'5 

] 

Z      5463 

1  8 

24  Feb  

53       346-67 

346.63 

0                         

26  Feb  

59       346.24 

346.20 

o                  .... 

A    10508]     4.5 

W  (3.3)-  B  (2.2), 

26  Feb  

60  i   1657.03 

1665.07 

1083.17     D    10473  \     4.5 

X  (6.2),  S(y.  2), 

Z    i  0422  j     8 

Y(I2) 

27  Feb  

61  |    346.45 

346.41 

0                         

A     7895 

4-5 

B  (3.  2),  W(2.3), 

i  Mar  

62      1388.84 

1394  13 

853.76     D     7869 

4-5 

X  (6.2),  S  (7.2), 

Z     7829 

8 

Y  (.2) 

A     9086 

4-5 

Do. 

i  Mar 

6}       1  5  M   67 

i  520.  20 

960.29     D     9055 

4   5 

^j       '  ?  *  j  •  **/ 

Z      9010 

8'5 

A    10265 

4-5 

Do. 

i  Mar 

64     1632  03 

l6}9  78 

1062  .15     D    10229 

4-  5 

v*r    j    •  vj*  •  vj 

1  ^*J\7  •  /u 

Z    10178 

]  8 

A    10511]     4.5 

Do. 

i  Mar  

65    '655-77 

1663.81 

1082.84     D    10474^  i  4.5 

. 

Z    10420]  i  8 

2  Mar  

66       346.20 

346-I7 

o                  .... 

3  Mar  

67     1386.28 

'39'-55 

A      7885]      4-5 
852.44     D     7861       4.5 

W  (3-3).  B  (2.  2) 

X  (6.2),  8(7.2) 

Z      7820]     8 

Y  (12) 

EXPERIMENTAL   DATA   AND   CALCULATED   RESULTS. 

TABLE  XIII. —OBSERVED  GAS-THERMOMETER  DATA — Continued. 
Gas  Filling  No.  2 — Continued. 


75 


Date. 

1909. 

No. 

f  (or  />„') 

P  (or  />„) 

t 

Standard        Posi-          Other  elements 
elements.        tjon            and  positions. 

A 

9088]      4 

.5       W  (3.3),   5(2.2), 

3  Mar  

.    68 

'5" 

95 

1518.48 

959 

81 

D 

9059       4 

.5     *  (6.2),  5(7.2), 

Z 

9013       8 

Y   (.2) 

A 

10257 

4 

.5                 Do. 

3  Mar  

.    69 

1628 

7' 

1636.46 

1060 

24 

D 

I  O22  I  >      4 

•5 

Z 

10169!     8 

A 

10512]      4 

.5                 Do. 

3  Mar.... 

•    70 

1654 

46 

1662.50 

1082 

73 

D 

10478?     4 

•5 

Z 

10444]      8 

5  Mar.  .  .  . 

7i 

345 

98 

345-94 

0 

Gas  Filling  No 

3- 

1909. 

4  June  .  .  . 

•    72 

345 

31 

345  27 

o 

A 

3403 

4 

.1      Y(i),  a  (2.4), 

E 

34'9 

4-3     b  (6.4) 

4  June  .  . 

73 

86  1 

67 

862.94 

4«7 

07 

F 

34'4f     4 

-5 

G 

3416       4 

•7 

Z 

3370 

8 

A 

5516 

4 

.  i                Do. 

E 

5535 

4 

.3 

4  June  .  . 

•    74 

i  i  18.  50 

i  120.83 

629 

.  1  1 

F 

5528 

4 

-5 

G 

5529 

4 

•  7 

Z 

5461 

8 

5  June  .  .  . 

•    75 

345 

3' 

345  27 

o 

A 

9090' 

4 

.  i                 Do. 

E 

9114 

4 

-3 

5  June  .  .  . 

•    76 

1510 

50 

1515.27 

959 

•77 

F 

9099 

4 

•  5 

G 

9108 

4 

-7 

1 

Z 

9OO2( 

8 

i 

A 

10258* 

4 

.1                 Do. 

E 

10285 

4 

•  3 

5  June  .  .  . 

•    77 

1628 

08 

1633.64 

1060 

53 

F 

10266 

4 

.5 

G 

10279 

I  4 

•  7 

Z 

10161 

8 

A 

10503!      4 

.1                Do. 

E 

10529]     4 

•  3 

5  June  .  .  . 

.    78 

1652 

36 

1658.  10 

1081 

.28 

F 

10510 

4 

•5 

G 

10523       4 

-7 

Z 

10404]      8 

7  June.  .  . 

79 

345 

50 

345-46 

O 

;     10  June.  .  . 

.    80 

345 

52 

345.48 

o 

F 

9120 

4 

.1     a  (i),6(2.4), 

E 

9128 

.3     c(6.4),«(7.3) 

18  June.  .  . 

.    81 

1512 

.96 

1517.69 

961 

.21 

A 

9080 

4 

-5 

G 

9122 

;  4 

•7 

Z 

9015 

8 

F 

10299 

4 

.1                 Do. 

j 

E 

10300 

,  4 

•3 

18  June.  .  . 

.    82 

1630 

94 

1636.53 

1062 

53 

A 

10252 

4 

-5 

G 

10292 

i    A 

•  7 

Z 

10181 

8 

F 

10534 

4 

.1                 Do. 

E 

10534 

4 

3 

18  June.  .  . 

..    83 

1653 

.61 

1659.37 

1082 

•'4 

A 

10487?     4 

5 

G 

10526        -\ 

-7 

Z 

1  0403  J      £ 

76 


HIGH   TEMPERATURE   GAS   THERMOMETRY. 


TABLE  XIII. — OBSERVED  GAS-THERMOMETER  DATA — Continued. 
Gas  Filling  No.  3 — Continued. 


1 
Date.              No 

P'  (or  />„') 

P  (or  pa) 

i                     j 

Standard        Posi-          Other  elements 
elements.         tion.            and  positions. 

1909. 

F    10536 

4-' 

a  (i),b  (2.4) 

18  June  84 

1654.51 

1660.27      1082.91 

E    10534 
A    10485 

4.3     c  (6.4),  e  (7.3) 
1  4-5 

G   10525 

4-7 

Z    10426 

8 

19  June  85 

345-5" 

345-47 

0 

Gas  Filling  No 

33. 

1909. 

I 

~ 

1  9  June  86 

219.73 

219.71           o 

.... 

F     5520 

4   " 

a  (i),e  (2.3) 

E     5520 

;  4-3 

c  (6  .  3  )  ,  /  (7.3) 

19  June  87 

710.34 

711  .83       627.61 

A      5484 

4-5 

Z      5437 

r 

F     9139 

4-  l 

Do. 

E    9136 

43 

19  June  88 

962  .  2  1 

965.23       961.71 

A     9089 

4-5 

G     9131 

4-7 

Z     9036 

8 

F    10540 

4.1  |              Do. 

1 

E    10538 

4-3 

1  9  June  89 

1051.74 

1055.41  |  1082.75 

A    i  0490 

'  4-5 

G    10532 

!  4-7 

. 

Z    10428 

8 

2  1  June  90 
22  June  92 

219.74 
220.65 

219.72 
220.63 

O 

o 

24  June  93 

220.62 

220.59 

o 

25  June  95 

220.56 

220.53 

o 

H    ,4251 

4.  i                 Do. 

E    14227 

4-3 

2  July  96 

1283.36 

1288.82 

1391.97 

F    14222 

i  4-5  : 

G   14245 

i  4-7  j 

Z    14121 

8 

I 

H    14282 

4.1  !              Do. 

E    14247 

4-3 

2  July  97 

1285.43 

i  290  .  89 

1394.89 

F    14241 

4-5 

G    14274 

4-7 

Z    14156 

8 

3  July  98 

22  I  .  02 

220.99 

o 

H    14213 

4-  " 

Do. 

E    14214 

4-3 

3  July  99 

1  28  1  .  97 

1287.45      1393  34 

F    14196 

4-5 

G    14216 

4-7 

Z    14099 

8 

H    .4264 

4.1                 Do. 

E    14242 

4-3 

3  July  100 

1284.05 

1289.54      1396-17 

F    14235 

4-5 

G    14259 

4-7 

Z    14156; 

6  July  101 

22O  .  62 

220.60          o 

EXPERIMENTAL   DATA   AND   CALCULATED   RESULTS. 

TABLE  XIII. — OBSERVED  GAS-THERMOMETER  DATA — Continued. 
Gas  Filling  No.  4. 


77 


Date. 
1909 

8  July... 

Xo. 

.  .  .  102 

P'  (or  p,,') 
2l6.8l 

P  (or 

216 

Po) 

79 

t 
o 

Standard 
elements. 

H  14235 

Posi-    Other  elements 
tion.    and  positions. 

4.1  a  (i),  e  (2.3) 

E 

14216 

4 

3  c  (6.  3),  7  (7-  3) 

8  July... 

.  .  .  I03 

I26l 

35 

1266 

80 

I39L15 

F 

14209 

4 

5 

G 

14222 

4 

7 

Z 

14124 

8 

H 

14249 

4 

i       Do. 

E 

14229 

4 

3 

8  July... 

104 

1263 

'3 

1268 

59 

1393-55 

F 

14199!-  4 

5 

G 

14236   4 

7 

Z 

i4'55J   8 

9  July.. 

.  •   105 

217 

36 

217 

33 

o 

II 

14251 

4 

i       Do. 

E 

14236 

4 

3 

9  July... 

..  106 

I26l 

7  ' 

1267 

1  5 

1391.64 

F 

14233 

4 

5 

G 

14241 

4 

7 

Z 

14123 

8 

H 

14240 

4 

i       Do. 

E 

14236 

4 

3 

9  July... 

.   107 

1263 

01 

1268 

46 

'393  44 

F 

14225 

4 

5 

G 

14233 

4 

7 

Z 

14152]   o 

10  July.  .  . 

.  ..  108 

2I7 

35 

217 

33 

o 

H 

15019 

4 

i       Do 

E 

15020 

4 

3 

10  July... 

109 

,306 

60 

1312 

52 

'455-37 

F 

4 

5 

G 

4 

7 

Z 

14903 

8 

12  July... 

.  .  .110 

217 

36 

217 

34 

o 

H 

14978 

4 

i       Do. 

E 

14980 

4-3 

12  July.  .  . 

.  .  .  in 

1305 

53 

1311 

35 

1453-52 

F 

4 

5 

G 

4 

7 

Z 

14867 

8 

i 

H 

14980 

4 

i       Do. 

E 

14960 

4 

3 

12  July.  .  . 

..112 

1305 

46 

1311 

28 

'453  3' 

F 

I4947i-  4 

5 

G 

4 

7 

Z 

14872]   8 

13  July.. 

.  .  .  113 

2I7 

40 

217 

38 

0 

10  Sept.  . 

.114 

217.38 

217 

36 

o 

II 

1   4 

i  u  (i),  c  (2.3) 

1  1  Sept.  . 

.  .  .  II5   1328.68 

1334 

79 

1484.70 

E 
F 

i  5389   4 
'5374   4 

3  e  (6.  7),  7  (7-3) 

5 

G 

4 

7 

A 

15357 

8 

II 

4 

i       Do. 

E 

15411 

4 

3 

11  Sept.  . 

...116 

1332 

18 

1338 

32 

1  489  .  60 

F 

15417 

4 

5 

G 

15418 

4 

7 

A 

15421 

8 

13  vSept.  . 

..117 

217 

62 

217 

60 

o 

H 

4 

i   a  (i),  c  (2.3) 

E 

'539' 

4 

3  *  (6.7)  7  (7-3) 

15  Sept.. 

...  118 

1329 

92 

1336 

03 

1487.36 

F 

'5389 

4 

5 

G 

'5399 

4 

7 

A 

15382 

8 

16  Sept... 

.  .  .  119 

217 

5' 

217 

49 

o 

78  HIGH   TEMPERATURE   GAS    THERMOMETRY. 

TABLE  XIII. — OBSERVED  GAS-THERMOMETER  DATA — Continued. 
Gas  Filling  No.  4 — Continued. 


Date               No 

.    p'_(or  p0')      p  (or  po>             t 

Standard 
elements. 

Posi- 
tion. 

Other  elements 
and  positions. 

H       .... 

4-1 

a  (i),  c(2.3) 

1909 

E   15386 

4-3 

g  (6.  7),  /  (7  .3) 

17  Sept  I2C 

1329.68      1335  .78      1486.95 

F    15376 

4-5 

G    15368 

4-7 

A    15379 

8 

H     .... 

4-1 

Do. 

E    '5397 

4-3 

17  Sept  |i2i 

1331.40      1337.51       148934 

F    15396 

45 

G    15389 

4-7 

A    15412] 

8 

18  Sept  122       217.52       217.50!        o 

H      .... 

4-  1 

Do. 

E    1499' 

4-3 

21  Sept  123      1306.75      1312.72      1454.83 

F    ,4996 

4-5 

G    14957 

4-7 

A    .4982 

8 

H      .... 

4-  1 

Do. 

E   14979 

43 

21  Sept  124     1307.28     1313.25      1455.60     F    14984 

4-5 

G   14952 

4-7  1 

A    14996 

8 

22  Sept  125       217.45       217.43          o 

H    10618 

4.  i      a  (1.5),  J  (2.4) 

E    10626 

4.3  ;  c  (6.2),  e  (7.2) 

27Nov'  126      1045.80      1049.49      1090.  59  ;F    10622 

4-5 

G    10616 

4-7 

C    10567]      8 

29  Nov  127       217.28       217.26           o 

H   12002}     4.  i                 Do. 

E    12006 

i  4-3 

9  Dec  128     1  129.  52      1  133.91      1206.63 

F    12003 

45 

G     12OIO 

1  4-7 

C    11914 

8 

H    13106 

!  4.1                 Do. 

E    13112 

!  4-3 

9  Dec  129      1194.81      1199.74      1298.01 

F    13107 

4-5 

G    13115 

4-7 

j 

C    13007 

8 

H   14246 

4.1                 Do. 

E   14250 

4-3 

9  Dec  130     1261.16     1266.68     1391.45     F    14248 

'     4-5 

1  G   14256 

4-7 

C    14146]     8 

10  Dec.  .  .  .  .  .  131        217.30       217.28           o 

H    i  i  940 

4.'     od),  J  (2.3). 

E   11946 

4.3     c  (6.2),  e  (7.1) 

20  Dec  132      1  125.92      1  130.29     1  201  .50     F    1  1951 

4-5 

G    11949 

4-7 

C    11887 

8 

H   14950 

4.1                 Do. 

E   14958 

43 

20  Dec  133      1302.40     1308.33      1450.03     F    14962 

4-5 

G   14955 

4-7 

C    14882 

8 

H   16156 

4.1                 Do. 

E    16160 

4.3 

20  Dec  134      1372.16     1378.78     1550.15     F    16170 

4-5 

G  16148 

4-7 

C   16075 

8 

21  Dec  135       217.29       217.27            o               

1  Outside-wound  furnace.     See  p.  56  and  Fig.  10. 


THE   TRANSFER   TO   THE    FIXED   POINTS.  79 

14.  THE  TRANSFER  TO  THE  FIXED  POINTS. 

After  the  thermo-elements  are  removed  from  the  bulb,  their  E.  M.  F. 
at  the  fixed  points  must  be  determined  by  immersing  them  in  melting  or 
freezing  metals  or  salts.  The  instrumental  corrections  to  the  readings  so 
obtained  were  the  same  as  in  the  case  of  the  gas- thermometer  readings. 
The  error  due  to  contamination  was  also  present  above  1 100°,  just  as  in  the 
gas-thermometer  furnace,  and  was  a  very  disturbing  factor  in  determining 
the  melting-points  of  nickel,  cobalt,  and  palladium.  Its  source,  however, 
was  not  usually  iridium  vapor  from  the  furnace  or  rhodium  from  the  wire 
of  the  element,  but  was  either  vapor  of  the  melting  metal  itself,  or  (when 
a  hydrogen  atmosphere  was  used)  the  products  of  reduction  of  silica.  In 
the  presence  of  hydrogen,  silica  rapidly  deteriorates  platinum  wire  by  reduc- 
tion and  alloying,  as  has  been  shown  in  this  laboratory  by  Shepherd,1  and 
elsewhere  by  several  observers.  The  contamination  can  be  partly  prevented 
by  the  use  of  a  glazed  procelain  tube  surrounding  the  thermo-element, 
instead  of  an  unglazed  magnesia  tube;  but  an  additional  uncertainty  is 
thereby  introduced  through  the  contamination  of  the  melting  metal  by  the 
melted  glaze  on  the  porcelain.  For  this  reason  nickel  and  cobalt  did  not 
prove  to  be  as  satisfactory  fixed  points  as  had  been  hoped,  since  it  was 
necessary  to  melt  them  in  an  atmosphere  of  hydrogen.  Palladium,  however, 
can  be  melted  in  the  open  air  and  serious  contamination  by  silicon  thus  be 
avoided,  although  the  palladium  itself  gradually  contaminates  the  wire. 

Above  1 100°  it  was  found  better  to  make  direct  comparisons  of  all  the 
elements  with  one  or  two  whose  fixed  points  had  been  determined,  rather 
than  to  contaminate  them  all  by  a  direct  determination.  For  making  these 
comparisons,  the  plan  first  used  was  to  bring  a  crucible  of  molten  silver  to 
a  constant  temperature  and  insert  the  elements  (protected  by  a  glazed 
Marquardt  porcelain  tube)  successively  into  the  silver  bath.  There  is  an 
uncertainty,  however,  in  these  measurements,  of  2  to  3  microvolts  caused 
by  small  differences  of  temperature  within  the  tube  and  the  slight  cooling 
produced  by  introducing  cold  wires  into  the  furnace.  A  better  method  is 
to  join  together  the  two  platinum  wires  and  the  two  alloy  wires  of  the 
elements  to  be  compared,  and  determine  the  small  E.  M.  F.'s  of  each  pair 
at  several  temperatures,  from  which  the  difference  between  the  elements  at 
those  temperatures  can  be  obtained  by  algebraic  addition.  This  method 
offers  a  great  advantage  in  that  the  temperature  need  be  only  approximately 
constant  and  approximately  known,  since  the  differences  in  most  cases 
amount  to  only  a  few  microvolts.  By  this  method  the  comparison  can  be 
very  quickly  made  at  1500°  in  the  blast-lamp  flame,  which,  with  a  little  care, 
can  be  made  to  give  a  temperature  constant  to  20°. 

All  the  metal  melting-points  here  described,  except  that  of  palladium, 
were  made  in  an  upright  cylindrical  furnace  through  which  passed  a  glazed 
porcelain  tube  which  could  be  tightly  closed  above  and  below  and  therefore 
permitted  the  atmosphere  about  the  melting  metal  to  be  perfectly  con- 
trolled. An  effort  was  first  made  to  accomplish  this  by  placing  the  entire 
furnace  inside  a  gas-tight  bomb  in  which  the  atmosphere  could  be  similarly 
varied,  but  the  persistent  retention  of  gases  by  the  various  clay  insulating 

'Am.  Jour.  Sci.  (4),  28,  p.  300,  1909. 


80  HIGH   TEMPERATURE   GAS   THERMOMETRY. 

materials  used  about  the  furnace  made  this  method  slow,  cumbersome,  and 
very  uncertain  in  its  results.  The  only  success  which  these  bomb  furnaces 
attained  was  to  permit  melting-points  to  be  measured  in  an  approximate 
vacuum  (about  i  mm.  pressure).  But  it  has  since  been  found  so  much 
simpler  to  operate  with  a  neutral  or  reducing  atmosphere  in  the  closed  tube 
passing  through  the  heated  zone  that  the  vacuum  furnace  has  not  been 
used  for  this  work. 

The  chief  disadvantage  in  the  use  of  a  tube  of  this  kind  is  its  effect  upon 
the  temperature  gradient  along  the  furnace  axis.  More  heat  is  diverted 
toward  the  ends  of  the  furnace  and  the  central  constant-temperature  zone 
becomes  shorter.  It  offers  no  difficulty  except  that  greater  care  must  be 
taken  in  locating  the  crucible  within  the  constant-temperature  region. 

The  qualities  desired  in  fixed  thermometric  points  for  establishing  and 
reproducing  a  scale  are : 

(1)  Exact  reproducibility  of  the  temperature  in  repeated  determinations 
with  the  same  charge  of  material  and  with  a  different  charge  independently 
obtained.     This  means  that  the  metal  or  salt  must  be  obtainable  either 
perfectly  pure  or  with  a  constant  amount  and  kind  of  impurity. 

(2)  Independence  of  particular  experimental  arrangements.     The  melt- 
ing-point of  a  metal,  for  instance,  must  be  sharp  and  definite  enough  so 
that  with  different  kinds  of  furnaces  and  different  rates  of  heating  the  same 
temperature  will  be  obtained. 

(3)  Convenience  and  safety  of  manipulation.     A  melting-point  which 
can  only  be  obtained  by  the  use  of  elaborate  experimental  arrangements 
is  undesirable,  even  though  it  be  reproducible  and  sharp.     Furthermore, 
the  substance  must  not  injure  the  instrument  to  be  calibrated. 

1 .  Reproducibilty. — No  extensive  experiments  have  been  made  in  the  pres- 
ent work  to  test  a  large  number  of  samples  of  different  origin.    It  appeared 
sufficient  to  assure  ourselves  that  all  of  the  metals  here  used  are  obtainable 
in  such  degree  of  purity,  or  with  such  a  constant  amount  of  impurity,  that 
the  variations  in  their  melting-points  are  well  within  the  limits  of  error  in 
the  scale  itself.     Waidner  and  Burgess"  have  recently  made  comparisons 
of  various  samples  of  pure  zinc,  antimony,  and  copper,  and  have  found  no 
differences  exceeding  0.3°. 2    Our  experience  has  been  the  same.    All  of  the 
metals  in  the  present  investigation  are  readily  obtainable  from  the  ordinary 
sources  of  supply.    They  have  been  carefully  analyzed  in  this  laboratory 
by  Dr.  E.  T.  Allen,  and  the  results  are  given  on  page  85. 

2.  Independence  of  Experimental  Conditions, — A  number  of  experiments 
were  made  to  test  the  effect  of  different  experimental  arrangements  on  the 
points.     Two  different  furnaces  were  tried,  one  65  mm.  inside  diameter 
and  150  mm.  long,  the  other  55  mm.  inside  diameter  and  230  mm.  long. 
The  region  of  constant  temperature  in  the  second  furnace  was  longer  than 
in  the  first  and  accordingly  there  was  a  larger  range  in  which  the  crucible 
could  be  moved  about  without  affecting  the  temperature.     This  furnace 
was  used  for  all  work  after  March  6,  1909.    The  ultimate  test  was  always 
the  agreement  between  the  melting  and   freezing  points.     Any  serious 
disagreement  of  these  two  in  metals  shows  that  some  influence  is  entering 
from  without. 

'Phys.  Rev.,  27,  467-469,  1909.  Bull.  Bur.  Stds.,  6,  149-230,  1909. 

-In  the  case  of  antimony,  this  statement  applies  only  to  Kahlbaum's  metal. 


THE   TRANSFER   TO   THE    FIXED    POINTS.  8 1 

The  results  of  the  study  were  briefly  as  follows :  (i)  The  best  dimensions 
for  a  charge  of  metal  are  about  25  mm.  diameter  by  45  mm.  deep.  (2)  The 
thermo-element  tube  should  be  about  5  mm.  above  the  bottom  of  the 
crucible.  (3)  There  is  a  region  within  the  furnace  in  which  the  melting 
and  freezing  points  agree  and  are  independent  of  the  rate  of  heating  or 
(within  limits)  of  the  depth  of  immersion  of  the  thermo-element;  it  is 
necessary  to  find  this  position  of  the  crucible  by  trial.  With  this  position 
once  determined,  the  temperature  of  the  zinc,  antimony,  silver,  gold,  and 
copper  points  can  be  relied  upon  within  0.2°.  With  large  charges  and  facili- 
ties for  stirring  the  metal,  Waidner  and  Burgess  have  found  the  zinc  point 
to  be  reproducible  in  a  given  furnace,  with  a  given  sample,  within  less 
than  0.1°. 

White1  showed  that  the  temperatures  of  the  two  silicate  points  used  for 
for  the  present  scale  are  reproducible  within  1.0°  independently  of  the 
dimensions  of  the  furnace  or  the  rate  of  heating.  For  a  mineral  melting- 
point  the  charge  should  be  small  (about  3  grams),  the  heat  should  flow  into 
the  thermal  junction  from  the  side  and  not  from  the  ends,  and  a  position 
in  the  furnace  should  be  found  in  which  the  melting-point,  determined  by  a 
bare  thermo-element,  does  not  vary  with  the  rate  of  heating. 

The  possibility  has  been  several  times  suggested  that  the  temperature 
of  the  thermo-element  inside  of  the  tube  might  possibly  be  lower  by  a  small 
constant  amount  than  the  metal  outside  of  the  tube,  and  that  this  error 
might  not  be  brought  to  light  by  such  experiments  as  have  been  described. 
Several  melting  and  freezing  points  of  copper  were  therefore  determined 
by  inclosing  the  entire  thermo-element  wire  in  a  thin  capillary  of  silica 
glass  which  was  slipped  over  the  wire,  bent  double,  and  melted  down  upon 
the  wire  at  the  junction  by  heating  in  the  oxy-hydrogen  flame.  This  was 
dipped  directly  into  the  molten  copper  to  within  5  mm.  of  the  bottom,  so 
that  there  was  practically  no  possibility  that  the  temperature  of  the  junction 
could  be  lowered  by  radiation  or  conduction  upward.  The  melting-point 
on  element  D  obtained  in  this  way  was  10,473  microvolts  as  compared  with 
10,473  microvolts  in  the  closed  glazed  tube.  There  appears  to  be  no  error 
from  this  cause. 

3.  Convenience  and  Safety  of  Manipulation.- — Zinc  and  gold  are  the  most 
convenient  of  manipulation,  as  they  require  no  special  atmosphere  and  the 
temperatures  are  easily  reached.  Antimony,  silver,  and  copper  require  an 
atmosphere  of  carbon  monoxide  and  are  somewhat  less  convenient.  More 
care  needs  to  be  taken  with  copper  than  with  silver  and  antimony  because 
of  the  considerable  effect  of  a  very  small  amount  of  oxide.  Antimony, 
silver,  gold,  and  copper  were  all  melted  in  carbon  monoxide,  made  by  drop- 
ping formic  acid  into  warm  sulphuric  acid,  and  purified  by  passage  through 
sodium  hydroxide,  lead  nitrate,  and  sulphuric  acid.  The  lead  nitrate  was 
introduced  to  make  certain  that  no  trace  of  hydrogen  sulphide,  which 
might  be  formed  if  the  acid  became  too  dilute  or  too  warm,  could  pass  into 
the  metal. 

The  two  silicates  (diopside  and  anorthite)  and  palladium  were  melted  in 
air.  The  silicate  points  are  very  convenient  to  arrange  and  manipulate, 
provided  the  furnace  is  well  insulated  so  that  the  temperature  can  be  reached 

'Diopside  and  its  relations  to  calcium  and  magnesium  metasilicates,  Am.  Journ.  Soc.  (4),  27,  p.  4,  1909. 


82  HIGH  TEMPERATURE  GAS   THERMOMETRY. 

without  difficulty.  Palladium  strains  the  platinum  resistance  furnace  near 
to  its  limit  of  endurance  on  account  of  the  high  temperature,  but  has  the 
great  convenience  of  not  requiring  a  reducing  atmosphere.  Special  pains 
need  to  be  taken,  however,  in  this  case,  to  protect  the  thermo-element  from 
contamination. 

Nickel  and  cobalt  were  melted  in  an  atmosphere  of  hydrogen  which  was 
made  by  electrolysis  in  a  large  glass  and  earthenware  generator,  and  purified 
by  passage  through  potassium  pyrogallate  and  sulphuric  acid.  Just  before 
the  thermo-element  was  introduced,  the  hydrogen  was  displaced  by  pure 
nitrogen  drawn  from  a  steel  tank  in  which  it  was  stored  under  pressure. 
The  supply  contained  a  trace  of  hydrogen  and  was,  therefore,  purified  by 
passing  over  hot  copper  oxide  and  through  calcium  chloride  and  sulphuric 
acid.  The  extreme  lightness  of  hydrogen  compared  with  the  outside  air 
(especially  when  it  is  heated  to  1450°)  makes  necessary  special  precautions 
in  order  to  keep  out  any  trace  of  air.  Furthermore,  hydrogen  always  caused 
contamination  in  the  thermo-element,  which  was  not  prevented  even  when 
the  hydrogen  was  replaced  for  a  short  time  during  the  melting  by  pure 
nitrogen.  Nickel  and  cobalt  are,  therefore,  not  recommended  for  frequent 
use  in  the  calibration  of  thermo-elements,  if  the  two  points,  diopside  and 
palladium  (or  diopside  and  anorthite),  give  a  sufficient  calibration  for  the 
purpose  in  hand 

The  apparatus  used  for  the  melting-points  of  nickel  and  cobalt  is  shown 
in  section  in  Fig.  13.  The  top  of  the  large  porcelain  tube  (Marquardt,  glazed 
outside  only)  was  closed  by  a  sliding  cup  of  brass  in  which  the  thermo- 
element tube  and  two  others  for  introducing  hydrogen  were  fastened  by 
heating  the  cup  and  pouring  in  molten  solder.  The  porcelain  tube  extended 
far  enough  out  of  the  furnace  to  keep  the  brass  cup  cool.  A  groove  near 
the  base  of  the  cup  carried  a  piece  of  asbestos  cord  which  made  a  gas-tight 
joint  with  the  porcelain  tube  and  permitted  the  whole  to  be  raised  and 
lowered  without  moving  the  crucible  or  opening  the  top  of  the  tube.  Two 
diaframs  of  Marquardt  porcelain  above  the  crucible  also  prevented  any 
considerable  radiation  upward  to  the  brass  cup. 

In  zinc,  antimony,  silver,  gold,  and  copper,  the  thermo-element  was  pro- 
tected by  a  glazed  Marquardt  tube  of  5  mm.  inside  and  8  mm.  outside 
diameter.  In  the  case  of  antimony,  the  tube  was  further  protected  by  a 
thin  tube  of  graphite  which  fitted  into  the  cover  of  the  crucible.  With 
diopside  and  anorthite,  some  contamination  from  iridium  in  the  furnace 
may  take  place,  but  can  be  largely  prevented  by  surrounding  the  supporting 
tube  with  pure  platinum.  Here  the  thermo-element  dips  directly  into  the 
molten  silicate.  A  glazed  Marquardt  tube  can  not  be  used  with  the  silicates, 
for  the  glaze  flows  readily  at  these  temperatures  and  may  make  its  way  into 
the  charge.  With  nickel  and  cobalt,  glazed  Marquardt  tubes  and  also  pure 
magnesia  tubes  of  the  same  size  were  used,  but  neither  protects  the  element 
perfectly  from  contamination.  In  palladium  only  the  pure  magnesia  tubes 
were  used. 

Zinc,  antimony,  silver,  gold,  and  copper  were  melted  in  graphite  crucibles 
27  mm.  in  diameter  and  80  mm.  deep  inside,  and  37  mm.  in  diameter  and 
100  mm.  high  outside.  The  charge  of  metal  was  from  45  mm.  to  55  mm. 
deep.  Diopside  and  anorthite  were  melted  in  small  platinum  crucibles  10 


THE   TRANSFER   TO   THE   FIXED   POINTS.  83 

mm.  in  diameter  and  18  mm.  deep,  suspended  by  platinum  sleeves  from 
the  open  end  of  unglazed  Marquardt  tubes,  as  described  and  illustrated  in 
the  paper  already  referred  to.1 

Nickel  was  melted  in  an  unglazed  Marquardt  porcelain  crucible,  lined 
with  a  paste  consisting  of  about  90  per  cent  A12O3  and  10  per  cent  MgO; 
and  also  in  a  Berlin  "pure  magnesia"  crucible.  The  charge  was  about 


FIG.  13.  The  furnace  in  which 
the  cobalt  and  nickel  melting-points 
were  made,  showing  the  position 
of  the  metal  with  respect  to  the 
coil,  the  thermo-element  (T)  and 
the  arrangement  (H)  for  maintain- 
ing a  hydrogen  or  nitrogen  atmos- 
phere. 

25  mm.  in  diameter  and  30  mm.  deep.  Cobalt  could  not  be  melted  in  the 
alumina-lined  crucible,  as  the  metal  penetrated  through  the  lining  and 
attacked  the  porcelain.  It  was,  therefore,  melted  in  a  "pure  magnesia" 
crucible  made  by  the  Konigliche  Porzellan  Manufaktur.  The  material  of 
these  crucibles  probably  contains  a  small  percentage  of  silica.  Palladium 
was  melted  in  a  crucible  made  in  this  laboratory  from  a  specially  pure 
magnesia  made  by  Baker  and  Adamson.  The  magnesia  was  first  shrunk 
by  heating  to  a  temperature  higher  than  that  at  which  the  crucible  was  to 


'W.  P.  White,  Am.  Jour.  Sci.  (4),  28,  p.  477,  1909- 


84  HIGH  TEMPERATURE   GAS  THERMOMETRY. 

be  used,  and  was  then  made  into  a  paste  with  water  and  a  little  magnesium 
chloride,  spun  into  form,  and  baked. 

In  choosing  the  materials  for  such  determinations,  two  not  altogether 
concordant  standpoints  must  be  recognized:  (i)  The  materials  used  must 
be  of  absolutely  known  composition  and  of  high  purity  in  order  to  give  the 
melting-point  determinations  a  positive  significance;  (2)  the  same  mate- 
rials in  the  same  purity  must  be  easily  obtainable  by  other  investigators 
in  order  to  enable  the  results  to  be  conveniently  utilized  by  others  if  desired. 

Metal  melting-points  were  given  the  preference  over  pure  salts  which 
have  been  repeatedly  suggested  for  this  purpose,  (i)  on  account  of  the 
greater  sharpness  of  the  melting-point,  (2)  on  account  of  their  general 
availability  for  such  determinations,  and  (3)  because  of  the  now  very  gen- 
erally established  custom  of  comparing  the  results  of  different  observers 
through  the  medium  of  these  standard  melting-points.  The  metals  used 
in  this  investigation  were  from  various  sources,  which  will  be  specified 
below.  Each  has  been  very  carefully  described  and  analyzed  by  Dr.  E.  T. 
Allen  of  this  laboratory,  whose  report  is  printed  in  full  on  page  85  and 
following.  We  prepared  none  of  the  metals  ourselves.  Those  which  were 
used  were  purchased  from  firms  who  may  fairly  be  expected  to  supply  the 
same  nominal  quality  to  any  other  investigator  who  may  care  to  use  them, 
but  it  must  be  emphasized  in  this  connection  that  metals  furnished  under 
the  same  description  by  the  same  dealer  at  different  times  have  not  always 
proved  of  uniform  purity  and  probably  can  not  at  present  be  expected  to 
be  so.  The  variations  in  the  thermal  behavior  of  the  different  samples  is 
not  great,  never  amounting  to  more  than  i°  in  our  experience;  but  we  are 
of  course  unable  to  offer  any  guarantee  that  the  same  metals  obtained  in 
future  will  remain  within  this  limit,  nor  is  the  dealer's  guarantee  at  present 
a  sufficient  protection. 

As  the  situation  now  stands,  the  errors  in  the  gas-thermometer  measure- 
ments are  of  the  same  order  as  the  differences  between  the  melting-points 
of  different  samples  of  a  given  metal  obtained  at  different  times  from  the 
same  dealer  and  of  the  same  (nominal)  purity.  This  may  serve  to  empha- 
size more  than  ever  before  the  desirability  of  a  provision,  preferably  by 
some  national  bureau  of  standards,  for  standard  metals,  the  uniform  purity 
of  which  can  be  absolutely  depended  upon,  and  in  terms  of  which  such  con- 
stants can  be  expressed.  In  the  absence  of  such  a  provision,  it  is  difficult 
to  see  just  how  to  make  the  gas  scale  conveniently  available  for  general  use 
in  its  full  accuracy.  This  is  furthermore  a  matter  of  considerable  importance 
in  view  of  the  extended  extrapolation  to  which  the  gas  scale  is  frequently 
subjected  by  the  use  of  thermo-elements  or  otherwise.  Supposing  the  metal 
melting-points  to  be  capable  of  reproducing  the  temperature  curve  correctly 
within  i°  at  the  copper  point  (1082.6°),  an  extrapolation  to  1500°  may  easily 
remain  uncertain  by  as  much  as  10°  in  the  hands  of  different  individuals 
using  the  same  function  for  the  extrapolation. 


THE   METALS   USED.  85 

15.  THE  METALS  USED. 
(By  B.  T.  Allen.) 

The  object  of  these  analyses  was  primarily,  of  course,  to  decide  whether 
the  metals  should  be  used  or  rejected  for  the  temperature  scale,  and  those 
selected  were  examined  very  carefully,  so  that  in  the  future,  when  more  is 
known  about  the  specific  lowering  which  the  various  impurities  produce 
on  the  melting-point,  corrections  may  be  made  if  desirable.  The  methods 
used  in  these  analyses  are  given  in  so  far  as  it  has  been  deemed  necessary. 
Details,  especially  in  those  cases  where  well-known  procedure  is  followed, 
have  been  purposely  omitted. 

The  accuracy  of  the  determinations  can  not  be  stated  absolutely.  There 
is  of  course  the  possibility  of  increased  solubility  of  difficultly  soluble  com- 
pounds in  the  comparatively  concentrated  solutions  of  the  metals  from 
which  the  impurities  have  to  be  precipitated,  viz,  5  to  6  g.  in  250  cc.  volume. 
Also,  when  it  is  necessary  to  separate  the  bulk  of  the  metal  by  precipitation 
from  the  impurities,  as  it  sometimes  is,  one  can  not  be  sure  that  the  impurity 
sought  is  not  occluded  by  the  precipitates.  In  most  cases,  the  latter  source 
of  error  is  probably  the  more  serious.  Only  methods  worked  out  syntheti- 
cally with  materials  laboriously  prepared  could  decide  these  questions.1 
Large  quantities  of  metal,  25  to  100  g.,  were  generally  taken  for  analysis, 
and  since  the  impurities  were  weighed  to  the  tenth  of  a  milligram,  the 
results  are  generally  stated  to  the  ten-thousandth  of  i  per  cent.  This  does 
not  mean  that  the  results  are  considered  accurate  to  this  figure.  The  varia- 
tion in  successive  determinations  comes  in  the  thousandths,  so  that  the 
fourth  decimal  place  may  have  about  as  much  value  as  the  second  in  an 
ordinary  analysis.  Great  pains  have  been  taken  to  purify  precipitates,  often 
by  many  precipitations,  so  that  in  all  cases  the  figures  given  may  be  regarded 
as  minima.  In  all  cases,  too,  I  have  endeavored  to  avoid  missing  anything, 
by  repeating  every  process,  rejecting  no  precipitate  or  solution  until  it  was 
decided  that  nothing  more  was  to  be  gotten  from  it.  In  any  reasonable 
case  of  suspicion,  blank  determinations  were  made  with  the  reagents.2 

CADMIUM. 

Eimer  and  Amend's  "Cadmium  metal  sticks"  purchased  in  1904  was 
used.  Inasmuch  as  the  cadmium  melting-point  was  used  for  the  purpose 
of  extrapolation  only  (p.  116)  and  but  a  single  gas-thermometer  measure- 
ment made  upon  it,  less  care  was  necessary  in  the  analysis  of  it  than  in  the 
more  important  metals  which  follow.  The  details  are  accordingly  omitted. 
Its  analysis  is  given  on  page  86. 

'The  data  on  this  question  which  are  known  to  me  are  quite  meager.  An  interesting  instance  is  given  by 
Mylius  and  Fromm  (Z.  anorg.  Chem.  9,  144-147,  1895),  using  a  specimen  of  zinc  in  which  they  could  detect 
no  impurities.  Additions  of  only  o.i  mg.  of  lead,  cadmium,  or  mercury  to  a  solution  of  40  grams  of  this 
zinc  could  be  detected  qualitatively. 

2  After  considerable  experience  in  the  examination  of  these  "pure"  metals  the  writer  has  reached  the 
conclusion  that  a  lo-gram  portion,  in  the  great  majority  of  cases,  will  give  as  satisfactory  results  as  a  larger 
portion  and  with  far  less  labor. 


86  HIGH   TEMPERATURE  GAS  THERMOMETRY. 

ANALYSIS  OP  CADMIUM. 

As None 

Cu Trace 

Pb 0860 

Zn Trace 

Fe 0025 

Co None 

Ni None 

S 0005 


Total  impurities 0890  per  cent. 

ZINC. 

This  metal  was  obtained  in  the  form  of  sticks  from  the  firm  of  Eimer  and 
Amend.  The  method  of  Mylius  and  Fromm  was  followed  for  the  principal 
impurities.1  100  grams  were  dissolved  in  nitric  acid.  The  solution  was 
then  diluted  and  ammonia  was  added  until  the  zinc  at  first  precipitated 
was  entirely  redissolved.  Then  enough  hydrogen  sulphide  was  added  to 
throw  down  all  the  impurities  of  the  hydrogen  sulphide  and  ammonium 
sulphide  groups  together  with  considerable  zinc.  The  precipitate  was  fil- 
tered off  and  further  separations  were  made  as  usual. 

The  platinum  metals  and  gold  were  not  looked  for,  as  it  was  thought 
quite  improbable  they  would  be  present,  but  arsenic  and  antimony  were 
sought  for  by  Giinther's  method.2  This  consists  in  the  volatilization  of  the 
hydrides  of  these  metals  which  are  separated  from  the  hydrogen,  which  forms 
at  the  same  time,  by  passing  the  gas  through  silver-nitrate  solution.  A 
special  form  of  apparatus  was  used  which  consists  of  a  i -liter  round-bottom 
flask  with  long  neck  35  mm.  wide  at  the  top.  This  is  closed  by  a  glass 
stopper  in  which  are  sealed  a  small  glass  tube  passing  to  the  bottom  of  the 
flask  and  serving  to  fill  the  flask  with  hydrogen  and  to  replace  the  gases 
formed  in  the  experiment;  a  dropping  funnel  through  which  the  acid  used 
to  dissolve  the  zinc  is  introduced,  and  lastly,  an  upright  outlet  tube  sur- 
rounded by  a  small  condenser.  The  outlet  was  connected  with  a  wash 
bottle  containing  a  solution  of  silver  nitrate.  As  pure  zinc  dissolves  with 
difficulty  in  dilute  hydrochloric  acid,  the  metal  was  reduced  to  the  form 
of  shavings  by  the  aid  of  a  lathe.  Fifty  grams  of  these  shavings  were 
introduced  into  the  flask,  the  air  in  which  was  at  once  replaced  by  hydrogen. 
Dilute  hydrochloric  acid  was  then  let  in  through  the  dropping  funnel.  The 
solution  was  facilitated  by  warming.  At  the  end  of  the  operation,  the  gas 
in  the  flask  was  driven  out  by  pure  hydrogen.  The  silver-nitrate  solution, 
which  contained  a  black  precipitate,  was  then  filtered.  The  antimony  in 
the  precipitate  was  determined  by  dissolving  it  in  nitric  acid  with  the  addi- 
tion of  a  little  tartaric  acid,  precipitating  the  silver  with  hydrochloric  acid, 
evaporating  the  filtrate  to  dryness  on  the  steam  bath,  and  precipitating  by 
hydrogen  sulphide.  The  precipitate  was  dissolved  in  a  few  drops  of  ammo- 
nium sulphide,  the  solution  filtered  into  a  small  tared  porcelain  capsule, 
evaporated,  decomposed  by  nitric  acid,  and  weighed  as  Sb2O4.  After  sepa- 
rating the  silver  from  the  first  filtrate  which  contained  the  arsenic,  it  was 
evaporated  to  dryness,  reduced  with  sulphurous  acid,  and  precipitated  by 
hydrogen  sulphide.  None  was  detected  with  certainty. 


'Zeitschr.  anorg.  Chem.,  9,  144,  1895. 

-Lunge,  Chem. -tech.  Methoden  (1905),  ii.  322.    Zeitschr.  analyt.  Chem.,  20,  503-507, 


THE   METALS   USED.  87 

If  this  solution  had  been  tested  by  Marsh's  method,  no  doubt  a  trace 
would  have  been  found,  but  as  its  quantity  was  of  a  different  order  of  mag- 
nitude from  the  other  impurities  it  was  not  thought  worth  while  to  make 
the  test.  Giinther  determines  sulphur  at  the  same  time  with  arsenic  and 
antimony,  by  interposing  between  the  generator  and  the  absorption  cylinder 
which  contains  the  silver  nitrate  another  cylinder  containing  potassium- 
cadmium  cyanide,  which  absorbs  all  the  hydrogen  sulphide  and,  according 
to  him,  retains  no  arsenic  and  antimony.  Since  a  solution  of  this  cadmium 
compound  is  always  alkaline,  it  was  thought  safer  to  take  a  separate  portion 
of  zinc  for  the  estimation  of  sulphur,  silver  nitrate  being  used  as  the  absorp- 
tion reagent.  The  small  precipitate  was  examined  for  sulphur  by  dissolving 
in  nitric  acid  and  proceeding  as  usual.  Found  0.4  mg.  BaSO4.  Blank  gave 
0.3  mg.  BaSO4. 

The  zinc  was  tested  for  silicon  in  the  same  way  as  the  copper.     (See  p.  92.) 

ANALYSIS  OF  ZINC. 

Not  found 

.002 

Not  looked  for 

Not  looked  for 

Not  looked  for 

None 

None 

.051 

.004 

None 

None 

.006 

Si None 

S..  None 


As  
Sb 

Sn  

Au  
Pt 

Ag  
Bi.... 

Pb  

Cd  

Ni 

Co  

Fe.  . 

.063  per  cent. 
ANTIMONY. 

Twenty-five  grams  of  metal  were  powdered  in  an  agate  mortar  and  treated 
with  35  per  cent  HNO3  on  the  steam  bath.  As  soon  as  the  reaction  was 
practically  complete,  the  antimonic  acid  was  extracted  with  hot  dilute  nitric 
acid,  transferred  to  a  filter,  and  washed  with  water.  The  nitrate  and  wash- 
ings were  then  evaporated  to  dryness  with  hydrochloric  acid,  while  the 
antimonic  acid  was  digested  repeatedly  with  yellow  sodium  sulphide  till  the 
soluble  portion  was  dissolved.  The  residue,  after  a  little  washing,  was 
dissolved  in  nitric  acid,  evaporated  to  dryness,  freed  from  nitric  by  hydro- 
chloric acid,  and  the  chlorides  united  with  the  first  extract.  The  whole 
was  precipitated  by  hydrogen  sulphide.  The  washed  sulphides  were  then 
extracted  with  colorless  ammonium  sulphide.  From  this  solution  the  sul- 
phides were  thrown  down  by  acid,  filtered,  and  washed.  Then  they  were 
dissolved  in  hot  dilute  caustic  potash.  The  solution  was  boiled  with  perhy- 
drol  for  complete  oxidation,  and  arsenic  sought  for  by  Fischer's  method, 
viz,  reducing  by  ferrous  ammonium  sulphate  and  distilling  in  a  current  of 
hydrochloric  acid  gas.  No  As. 

A  separate  portion  of  5  grams  was  taken  for  tin.  McCay's  method1 
was  tried.  SnO2=  1.3  mg.  Sn=  i.o  mg.  =  o.o2  (?)  per  cent. 

A  separate  portion  of  25  grams  was  used  for  sulphur.  The  metal  was 
oxidized  by  nitric  acid  as  before,  and  the  soluble  portion  separated  and 

'Private  communication. 


88  HIGH  TEMPERATURE;  GAS  THERMOMETRY. 

evaporated.  The  residue  was  then  heated  with  a  small  excess  of  sodium 
carbonate  and  filtered.  The  residue  was  also  boiled  out  several  times  with 
sodium  carbonate  solution.  The  two  solutions  were  then  acidified  with 
hydrochloric  acid  and  treated  with  barium  chloride.  The  portion  soluble 
in  nitric  acid  gave  a  slight  precipitate,  which  was  further  purified,  after  the 
usual  washing  and  drying,  by  fusion  with  sodium  carbonate.  The  water 
extract  containing  the  soluble  sulphate  was  acidified  and  precipitated  a 
second  time.  BaSO,,  =  trace. 


ANALYSIS  OF  ANTIMONY. 


None 

.  .       0.02    (?) 

None 

.  .     Trace  (?) 
0.004 
None 
None 
None 

Co None 

Mn None 

Zn None 

Fe o .  007 

S Trace  (?) 


As. 
Sn. 
Ag. 
Pb. 
Cu. 
Bi. 
Cd. 
Ni. 


0.031  per  cent. 
ALUMINUM. 

Owing  to  the  difficulty  of  handling  this  metal,  small  portions  (10  grams) 
only  were  taken  for  analysis.  Heavy  metals,  except  arsenic  and  antimony, 
were  sought  for  in  the  hydrochloric  acid  solution  by  ordinary  methods. 
Only  a  trace  of  copper  was  found. 

For  phosphorus,  arsenic,  and  sulphur,  a  separate  portion  was  dissolved 
in  caustic  alkali  in  a  special  apparatus  entirely  of  glass.  The  vessel  was 
first  filled  with  purified  hydrogen  and  then  the  alkali  was  introduced  and 
the  gases  evolved  were  passed  through  silver-nitrate  solution.  At  the  end, 
the  gases  remaining  in  the  vessel  were  displaced  by  hydrogen.  The  precipi- 
tated silver  was  worked  over  for  the  different  elements.  No  As  or  Sb.  A 
separate  portion  was  used  for  sulphur.  BaSO4  =1.4  mg.  S  =  0.002  per  cent. 

To  determine  the  silicon,  10  grams  of  metal  were  dissolved  in  a  mixture 
of  nitric  and  sulphuric  acids,  using  a  platinum  dish.  With  hydrochloric 
acid  alone  nearly  all  the  silicon  is  lost  as  hydride.  The  brown  amorphous 
residue  was  filtered,  washed,  and  fused  with  sodium  carbonate.  From  the 
fusion  silica  was  obtained  in  the  usual  way.  SiO2  =  4i.4  mg.  81=0.194  per 
cent.  Repetitions  gave  0.189  per  cent  and  0.190  per  cent. 

For  the  carbon,  10  grams  of  metal  were  dissolved  in  NaOH  and  filtered 
through  glowed  asbestos,  washed  first  with  water,  then  with  dilute  acid, 
finally  with  water,  and  dried  at  105°.  The  asbestos  and  residue  were  then 
transferred  to  a  combustion  tube  and  burned  in  air  free  from  CO2.  The 
gases  were  passed  through  standard  Ba(OH)2.  A  considerable  precipitate 
was  obtained,  while  a  blank  gave  no  trace.  The  excess  of  Ba(OH)2  was 
then  titrated  with  standard  acid,  using  phenolphthalein  as  indicator.  5.05 
mg.  CO2  found.  0=0.014  per  cent.  A  duplicate  in  which  the  metal  was 
dissolved  in  KOH  gave  0.012  per  cent. 


THE   METALS   USED.  89 

For  the  iron,  10  grams  of  metal  were  dissolved  in  hydrochloric  acid,  and 
to  the  solution  was  added  tartaric  acid  free  from  iron.  From  this  solution 
the  iron  was  precipitated  by  colorless  ammonium  sulphide.  The  precipitate 
was  finally  changed  to  sulphate  and  determined  volumetrically.  Fe=4.6 
mg.  Blank  determination  gave  0.3  mg.  Fe  =  0.043  per  cent. 

Calcium,  sodium  and  potassium  were  sought  for  in  the  hydrochloric-acid 
solution,  by  precipitating  with  ammonia,  washing  the  large  precipitate,  and 
testing  the  evaporated  filtrate.  No  Ca.  Some  alkaline  chloride  was  found, 
but  a  blank  showed  that  it  came  from  the  ammonia,  as  there  was  only  a 
difference  of  1.6  mg.  between  the  chloride  of  the  blank  and  that  in  the 
determination.  No  Na  or  K. 

ANALYSIS  OF  ALUMINUM. 


As  

None 

Sb  

P 

Cu  

Fe  

Si  

C  

o  013 

S   .    . 

Ca  

None 

Na..    . 

K  

None 

0.251  per  cent. 
SILVER. 

This  metal,  as  well  as  the  gold,  was  prepared  by  Mr.  Eckfeldt  at  the 
Philadelphia  Mint.  A  block  weighing  about  100  grams  was  cut  from  a 
larger  brick  with  a  hard  cold  chisel  and,  after  cleaning,  transferred  to  a  large 
casserole  of  Berlin  porcelain  and  dissolved  in  a  slight  excess  of  nitric  acid. 
During  the  operation  the  dish  was  covered  with  a  watch-glass.  A  small 
black  residue  was  now  filtered  off  on  the  felt  of  a  large  porcelain  Gooch 
crucible,  then  washed  and  dried.  The  asbestos  of  the  felt  was  previously 
heated  to  redness.  The  residue  was  then  laid  iri  a  porcelain  boat  which 
was  slipped  into  a  combustion  tube  containing  copper  oxide  and  heated  in  a 
current  of  oxygen.  The  outflowing  gas  was  passed  through  a  very  dilute 
standard  solution  of  barium  hydroxide,  i  cc.  =  o.97  mg.  of  CO2,  in  which 
a  decided  white  precipitate  appeared  at  once.  The  excess  of  baryta  was 
then  titrated  with  standard  acid.  A  blank  determination  previously  made 
gave  no  precipitate  in  the  baryta  water.  This  determination  is  of  no  impor- 
tance as  regards  the  melting-point  of  the  silver,  since  the  metal  had  to  be 
melted  in  graphite,  but  considering  the  source  of  the  silver  and  its  unusual 
degree  of  purity  the  determination  may  be  of  some  interest.  What  re- 
mained of  the  residue  after  the  carbon  was  burned  was  extracted  with  aqua 
regia.  The  solution  was  evaporated  to  dryness  and  taken  up  with  hydrochloric 
acid,  and  the  gold  was  precipitated  by  sulphur  dioxide.  The  filtrate  from 
gold  gave  a  slight  black  precipitate  with  hydrogen  sulphide.  This  precipitate 
weighed  only  o.i  mg.  after  it  had  been  glowed  in  a  small  porcelain  crucible, 
but  it  remained  black,  dissolved  in  a  few  drops  of  aqua  regia  which  left  a 
yellow  stain  when  evaporated,  and  gave  a  very  strong  rose  color  when 
dissolved  in  water  and  tested  with  a  drop  of  potassium  iodide — all  character- 


9O  HIGH   TEMPERATURE   GAS   THERMOMETRY. 

istic  of  platinum.  It  was  suspected  that  a  trace  of  platinum  might  exist 
in  the  acid  used  to  dissolve  the  silver,  but  a  blank  test  on  the  same  quantity 
of  reagent  proved  the  contrary.  The  silver  solution  was  now  diluted  to 
several  liters  and  precipitated  with  hydrochloric  acid.  The  filtrate  was 
evaporated  in  porcelain  to  a  small  volume  and  in  this  the  remaining  impuri- 
ties were  sought  for  by  well-known  methods.  Only  lead  and  iron  and  the 
merest  trace  of  copper  were  found. 

A  blank  determination  was  made  for  iron.  Found  in  the  silver + reagents 
0.0013  Per  cent;  in  the  reagents,  0.0002  per  cent;  leaving  o.oon  per  cent 
in  the  silver. 

For  the  estimation  of  sulphur,  a  separate  portion  of  38  grams  was  taken, 
the  silver  was  removed  in  the  same  manner,  and  the  filtrate  evaporated  to 
dryness  in  porcelain.  The  small  residue  was  then  evaporated  again  with 
hydrochloric  acid  to  decompose  nitrates.  The  final  residue  was  dissolved 
in  a  small  volume  of  water  acidulated  with  hydrochloric  acid,  filtered  to 
remove  any  silver  chloride  which  might  have  escaped  precipitation,  and 
precipitated  with  barium  chloride.  Found  1.4  mg.  BaSO4,  while  the  same 
quantity  of  reagents  gave  0.4  mg.  BaSO4:  8=0.0004  per  cent. 

ANALYSIS  OP  SILVER. 

As None  Hg None 

Sb None  Cd None 

Sn None  Zn None 

Au 0005  Ni None 

Pt oooi  Co None 

Cu Mere  tr.        Fe oon 

Bi None  S 0004 

Pb 0008  C 0003 


00032  per  cent. 
GOLD. 

About  350  grams  of  "proof  gold"  were  obtained  from  the  Philadelphia 
Mint.  It  was  prepared  by  Mr.  Jacob  Eckfeldt.  A  sample  of  gold  prepared 
in  a  similar  manner  by  Mr.  Eckfeldt  was  used  by  Professor  Mallet  in  his 
determination  of  the  atomic  weight  of  this  metal.  The  method  of  purifica- 
tion is  given  in  the  Am.  Chem.  Jour.,  vn,  73,  1899.  Professor  Mallet  found 
no  systematic  difference  between  this  gold  and  two  other  samples,  one  of 
which  was  obtained  from  the  Mint  of  England,  and  the  other  of  which  was 
prepared  by  himself.  In  view  of  these  facts,  it  was  evidently  unnecessary 
to  analyze  the  gold. 

COPPER. 

The  copper  was  of  the  form  known  as  "copper  drops  cooled  in  hydrogen " 
and  was  obtained  from  Eimer  and  Amend  of  New  York.  Not  all  copper 
of  this  brand  is  equally  pure.  The  sample  analyzed  was  a  portion  of  a  25- 
pound  lot.  The  method  followed  in  the  analysis  was  essentially  that  of 
Hampe, '  in  which  the  copper  is  separated  from  the  impurities  by  precipita- 
tion as  cuprous  thiocyanate.  A  loo-gram  portion  was  placed  in  a  large 
casserole  of  Berlin  porcelain,  dissolved  in  nitric  and  sulphuric  acids,  and 

'Lunge,  Chem.-tech.  Methoden  (1905),  vol.  n,  202.  Chem.  Ztg.  17,  1691-1692,  1893. 


THE    METALS    USED.  91 

the  solution  was  then  evaporated  to  drive  off  the  excess  of  nitric  acid.  This 
troublesome  operation  can  be  greatly  facilitated  by  the  use  of  a  crown 
burner,  though,  as  dilution  and  evaporation  have  to  be  several  times  re- 
peated, small  losses  are  difficult  to  prevent.  Duplicate  determinations, 
however,  proved  that  they  were  entirely  negligible  as  regards  the  small  per- 
centage of  impurities.  The  sulphate  of  copper  was  now  dissolved  in  water 
and  diluted.  A  little  HC1  was  added  and,  after  standing,  the  solution  was 
filtered.  The  residue  left  on  the  filter  was  extracted  with  ammonia  to  remove 
silver  chloride  and  the  remaining  part  of  it  was  treated  with  aqua  regia. 
There  was  still  left  a  little  silica,  from  the  porcelain  dish  in  which  the  copper  was 
dissolved.  The  solution  obtained  by  aqua  regia  after  the  nitric  acid  was 
entirely  driven  out  by  hydrochloric  acid  was  tested  for  gold  by  sulphur 
dioxide.  There  was  no  precipitate  in  the  cold  even  after  long  standing, 
though  evaporation  caused  the  precipitation  of  about  0.5  mg.  of  black  metal. 
This  remained  black  on  heating,  dissolved  only  partially  and  with  difficulty 
in  aqua  regia,  and  with  sulphuric  acid  and  ammonium  nitrate  gave  a  faint 
blue  color.  These  tests  indicate  iridium,  though  there  was  too  little  to 
identify  with  certainty.  The  rest  of  the  solution  which  had  been  tested 
for  gold  was  precipitated  by  hydrogen  sulphide  and  the  precipitate  was 
filtered,  washed,  and  burned  in  a  porcelain  capsule.  It  formed  a  yellow 
chloride  with  aqua  regia,  gave  a  precipitate  with  ammonium  chloride  and  a 
very  strong  test  for  platinum  with  potassium  iodide.  This  platinum  did 
not  come  from  the  acids  used  to  dissolve  the  copper,  since  the  same  quan- 
tities were  very  carefully  tested  by  hydrogen  sulphide  after  nearly  the 
wrhole  portion  had  been  driven  off  by  heating  in  porcelain,  and  were  found 
to  contain  not  a  trace. 

The  solution  containing  the  copper  was  then  warmed  and  saturated  with 
sulphur  dioxide.  After  standing,  a  further  portion  of  silver  was  precipitated, 
filtered  off,  and  washed.  It  was  then  dissolved  in  a  little  nitric  acid,  pre- 
cipitated again  as  chloride,  and  added  to  the  main  portion  of  the  silver 
chloride,  which  was  dried  at  130°  and  weighed. 

The  solution  still  containing  the  copper  \vas  diluted  to  about  8  liters,  and 
from  it  all  but  a  small  portion  of  the  copper  was  precipitated  by  a  standard 
solution  of  potassium  thiocyanate,  i  cc.  of  which  was  equivalent  to  about 
50  mg.  of  copper.  The  thiocyanate  was  proved  to  be  free  from  heavy  metals 
by  a  test  with  hydrogen  sulphide.  The  small  amount  of  iron  which  it  con- 
tained was  separated  before  the  solution  was  standardized,  by  the  addition 
of  a  little  ammonium  alum  followed  by  ammonia.  The  solution  was  allowed 
to  stand  and  then  filtered  from  iron  and  alumina.  The  precipitation  of  the 
copper  was  done  very  gradually  with  constant  shaking  to  avoid  carrying 
down  the  impurities,  and  after  long  standing  was  filtered.  The  filtrate  was 
concentrated  to  a  small  volume  in  porcelain.  A  small  additional  precipitate 
which  came  down  in  this  process  was  worked  over  with  care  to  avoid  any 
possible  loss  of  impurities,  especially  lead,  though  no  metal  but  copper  was 
found  in  it.  The  filtrate  was  then  examined  as  usual. 

A  word  is  needed  in  reference  to  the  presence  of  zinc.  This  was  found  in 
every  sample  examined,  in  fact,  it  was  generally  the  chief  impurity.  It  was 
suggested  that  this  zinc,  or  at  least  a  part  of  it,  might  have  come  from  the 
large  flasks  of  Jena  glass  in  which  the  acid  solutions  of  the  copper  stood. 


92  HIGH  TEMPERATURE   GAS  THERMOMETRY. 

To  test  this  point,  a  sample  of  copper  in  which  had  been  found  0.089  percent 
of  zinc  was  tested  again.  In  this  determination  Jena  glass  was  entirely 
discarded.  The  zinc  found  was  0.091  per  cent.  As  these  results  agree 
within  the  limits  of  error,  it  is  evident  that  Jena  glass  under  these  conditions 
will  not  contaminate  solutions  with  zinc,  at  least  in  quantities  of  this  order 
of  magnitude.  For  the  determination  of  silicon  in  the  copper,  25  grams  were 
placed  in  a  platinum  basin,  dissolved  in  nitric  and  sulphuric  acids,  and  evap- 
orated over  a  crown  burner  to  white  fumes.  The  residue  was  dissolved  and 
filtered.  The  filter  was  burned  and  the  small  residues  tested  for  silica  by 
hydrofluoric  and  sulphuric  acids.  Since  it  was  feared  that  some  silica 
might  come  from  the  watch  glass  used  to  cover  the  platinum  dish  during 
this  operation,  a  blank  was  carried  out  with  the  reagents  under  the  same 
conditions.  Within  the  limits  of  error  none  was  found. 

For  the  estimation  of  sulphur  the  method  of  Lobry  de  Bruyn2  was  used, 
in  which  the  copper  is  separated  from  the  nitric  acid  solution  by  electrolysis. 
Twenty-five  grams  of  metal  was  dissolved  in  75  cc.  nitric  acid  diluted  with 
about  an  equal  quantity  of  water,  and  then  the  excess  of  acid  evaporated 
as  far  as  possible  on  the  steam  bath.  The  electrolysis  was  done  in  a  large 
platinum  basin,  which  served  as  a  cathode.  The  basin  was  covered  with  a 
glass  plate  pierced  to  admit  a  cylindrical  platinum  crucible  which  formed 
the  anode.  The  current  density  was  about  0.015  ^^m  .  After  a  time  it 
was  found  necessary  to  pour  off  the  solution  from  the  precipitated  copper 
and  remove  the  free  acid  by  another  evaporation.  A  repetition  of  this  oper- 
ation is  advisable.  The  filtrate  from  the  copper  is  evaporated  to  dryness 
in  porcelain  and  the  small  residue  of  nitrates  decomposed  by  hydrochloric 
acid.  The  final  residue  is  dissolved  in  acidulated  water  and  precipitated  by 
barium  chloride. 

Found  in  25  grams  copper  ........................    4.2  mg.  BaSO4 

Found  in  75  cc.  nitric  acid  ........................    0.6  mg.  BaSO4 

3.6  mg  BaSO4  =0.002  per  cent. 


ANALYSIS  OF  COPPER. 


As  ....................... 

Sb  ....................... 

Sn  ....................... 

Se  ........................ 

Te  ....................... 

Au  ....................... 

Pt  metals  ................. 

Ag  (separate  determinations) 


None 
None 
None 
None 
None 
None 
.001  1 

Bi  
Pb  
Cd  
Zn  
Ni  
Co  
Fe  

None 
None 
None 
0007 
None 
None 
0038 

.  0007 

Si  

None 

.ooo.s 

S... 

.0020 

0.0083  per  cent. 
NICKEL. 

Two  5o-gram  portions  of  Kahlbaum's  electrolytic  nickel  were  dissolved 
separately  in  measured  quantities  of  nitric  acid  and  then  carried  to  white 
fumes  with  excess  of  sulphuric  acid.  Both. portions  were  then  dissolved  in 
water  and  filtered.  There  was  a  small  dark  residue  which  was  washed 
thoroughly  and  extracted  with  aqua  regia,  leaving  a  little  silica  from  the 
dish.  The  yellow  chloride  obtained  was  freed  from  nitric  acid,  saturated 
with  SO2,  and  left  to  stand.  No  gold.  Changed  to  chloride  again  and 
tested  with  caustic  soda  and  H2O2.  Still  no  gold.  Acidified  and  repre- 


-R.  des  trav.  chitn.  de  Pays-Bas,  10,  125,  1891. 


THE    METALS   USED.  93 

cipitated  with  NH4C1,  a  characteristic  yellow  precipitate  was  obtained. 
Confirmed  by  dissolving  the  chlor-platinate  in  hot  water  and  precipitating 
by  hydrogen.  Pt=  2.3  mg.  =  0.0023  per  cent.  The  main  solution  was  then 
precipitated  by  H2S  (volume,  2  liters).  The  small  black  precipitate 
obtained  was  worked  over  for  gold  and  platinum  together  with  the  above. 

Other  heavy  metals  were  tested  for  in  the  ordinary  way.  0.2  mg.  PbSO4  = 
about  o.i  mg.  Pb.  Cu=52.3  mg.  =  0.0523  per  cent. 

Ammonium  Sulphide  Group. — The  voluminous  solution  was  now  freed 
from  hydrogen  sulphide  by  evaporation,  some  ammonium  persulphate  was 
added,  and  a  stream  of  air  passed  through  the  solution  for  some  time.  No 
manganese. 

Fe2O3=6.i  mg.,  after  repeated  precipitation.    Fe=4.2  mg. 

Repeated  efforts  were  made  to  separate  zinc  with  H2S  on  the  principle  of 
the  lower  solubility  of  ZnS  in  dilute  acids,  but  without  satisfaction.  First 
I  tried  to  precipitate  a  small  fraction  of  the  nickel,  hoping  to  get  all  the 
zinc  with  it.  The  volume  of  the  solution  was  about  5  liters.  But  unless  so 
much  acid  was  added  that  strong  doubts  were  entertained  of  recovering 
any  zinc  that  might  be  present,  the  fraction  of  the  nickel  precipitate  was 
far  too  great.  Again,  all  the  nickel  was  precipitated  and  the  precipitate  was 
digested  with  cold  10  per  cent  solution  of  hydrochloric  acid.  Here  one  had 
to  fear  either  the  failure  to  remove  the  zinc  or  the  removal  of  too  much 
nickel  to  handle  without  so  many  precipitations  that  a  small  quantity  of 
zinc  would  probably  be  lost.  It  is  doubtful  whether  we  have  any  method 
which  will  give  very  small  amounts  of  zinc  in  metallic  nickel. 

The  whole  solution  was  now  tested  for  cobalt  as  follows:  It  was  freed 
from  H2S  by  evaporation,  acidulated  with  HC1,  and  precipitated  by  a-ni- 
troso-/3-naphthol  in  50  per  cent  acetic  acid.  This  was  added  in  several  por- 
tions. After  long  standing  the  precipitate  was  collected  and  washed.  The 
voluminous  precipitate  was  very  cautiously  burned  in  a  capacious  porcelain 
crucible.  Much  tar  was  formed.  The  residual  oxide  was  dissolved  in  nitric 
acid  and  the  cobalt  was  separated  from  nickel  by  KNO2  in  the  usual  way. 
The  potassium  cobalto-nitrite  was  finally  decomposed  by  sulphuric  acid 
and  precipitated  electrolytically  from  ammoniacal  solution. 

Co=  101.4  mg.-f-4.9  mg.  recovered  from  filtrate  and  weighed  as  sulphate. 
Total  =0.1063  Per  cent. 

Fe  and  Co  were  also  determined  in  a  separate  10  g.  portion  of  metal. 
Fe2O3  =  o.7  mg-  Fe=o.49  mg.  =  0.0049  Per  cent.  Co=  10.3  mg.  =  0.1030 
per  cent. 

A  separate  lo-gram  portion  was  taken  for  sulphur.  It  was  dissolved  in 
nitric  acid  and  evaporated  on  the  water  bath.  This  solution  was  diluted  and 
precipitated  with  a  slight  excess  of  sodium  carbonate.  The  filtrate  was  just 
acidulated,  evaporated,  and  treated  with  barium  chloride.  No  precipitate. 

ANALYSIS  OF  NICKEL. 

Au None  Bi None 

Pt 0023  Cd None 

As None  Zn None  found 

Sb None  Co 1063 

Sn None  Mn None 

Pb oooi  Fe 0042 

Cu 0523  -  •  •     None 

o.  165  per  cent. 


94  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

COBALT, 

Two  25-gram  portions  of  Kahlbaum's  metallic  cobalt  in  the  form  of 
powder  were  dissolved  in  150  cc.  water+35  cc.  concentrated  H2SO4.  The 
analysis  was  quite  similar  to  that  of  the  nickel. 

In  the  H2S  group  were  found :  Cu  =  8.9  mg.  =  o.oi 78  per  cent.  PbSO4  = 
12.9  mg.  Pb  =  o.o  1 76  per  cent. 

In  the  (NH4)2S  group  manganese  was  tested  for  as  in  the  nickel.  None 
was  found.  Fe2O3=o.9  mg.  Fe  =  0.0006  per  cent. 

As  the  tests  for  Ni  and  Zn  were  unsatisfactory,  another  portion  of  25 
grams  was  dissolved  in  dilute  sulphuric  acid  and  precipitated  by  H2S. 

The  nitrate  from  the  sulphides  was  filtered  and  freed  from  excess  of  H2S 
by  evaporation.  Then  it  was  diluted  to  i  liter  and  divided  into  two  por- 
tions. Both  were  neutralized  by  sodium  carbonate.  In  the  one,  manganese 
was  sought  for  by  ammonium  persulphate.  In  the  other  nickel  was  looked 
for.  A  little  ammonia  was  added  and  then  an  alcoholic  solution  of  dimethyl- 
glyoxime.  A  precipitate  containing  much  cobalt  was  obtained.  This  was 
worked  over  for  nickel  but  none  was  found.  For  sulphur  the  method  used 
in  the  analysis  of  nickel  was  followed.  BaSO4=  14.4 mg.,  blank  =  5.1  mg., 
difference  =  9. 3  mg.,  8  =  0.013  per  cent. 

ANALYSIS  OF  COBALT. 

Ag None  Bi None 

Au None  Cd None 

Pt None  Zn None 

As None  Ni None 

Sb None  Fe 0006 

Sn None  Mn None 

Pb 0176  S 013 

Cu 0178 

0.049  per  cent. 

HERAEUS'S  PALLADIUM. 

The  palladium  was  naturally  suspected  to  contain  other  metals  of  the 
platinum  group.  It  is  well  known  that  the  separation  of  these  metals  is  a 
problem  of  unusual  difficulty.  The  plan  here  was  therefore  to  precipitate 
most  of  the  palladium  from  solution  as  one  of  its  characteristic  compounds 
and,  while  the  filtrate  was  reserved  for  impurities,  to  redissolve  and  again 
precipitate  the  metal  as  another  characteristic  compound.  In  this  way  it 
was  hoped  that  those  impurities  retained  by  the  first  precipitate  would  not 
be  occluded  by  the  second  The  sheet  metal  was  first  cut  into  shavings  on  a 
milling  machine  especially  cleaned  for  the  purpose.  Then  the  shavings  were 
boiled  a  short  time  with  dilute  hydrochloric  acid  to  remove  any  iron  from 
the  surface,  and  then  washed  and  dried.  After  an  unsuccessful  endeavor 
to  dissolve  the  palladium  in  nitric  acid  (insoluble  brown  hydroxide  (?) 
always  formed),  it  was  dissolved  in  aqua  regia  and  rid  of  nitric  acid  by  suc- 
cessive evaporations  with  excess  of  hydrochloric  acid.  It  was  then  dissolved 
in  dilute  hydrochloric  acid  and  diluted  further  to  about  one  liter.  Ammonia 
was  added  in  excess.1  A  precipitate  came  down  and  redissolved  on  warming, 
all  but  a  little  ferric  hydroxide,  which  was  filtered  off.  The  filtrate  was 
then  evaporated  again  to  about  250  cc.,  and  then  diluted  and  precipitated 


F.  Smith  and  H.  F.  Keller,  Amer.  Chem.  Jour..  14,  423,  1892. 


THE    METALS   USED.  95 

with  stirring,  by  dilute  hydrochloric  acid.  The  voluminous  precipitate  of 
PdCl2.  2NH3was  now  filtered  and  washed  on  a  Biichner  porcelain  funnel, 
using  suction.  The  nitrate  we  will  call  "solution  A. "  The  precipitate  was 
then  dried  and  ignited  in  a  large  porcelain  crucible.  The  resulting  metal 
was  dissolved  in  aqua  regia  and  freed  of  nitric  acid.  This  solution  was 
diluted  and  precipitated  by  potassium  iodide,  and  the  filtrate  ("solution  B") 
removed  as  above. 

From  solutions  A  and  B,  separately,  the  platinum  metals  were  first  re- 
moved by  long  boiling  with  ammonium  formate.  The  metal — i  to  2  grams 
in  weight,  mostly  palladium — was  filtered  and  the  filtrate  and  washings 
were  examined  further  for  other  heavy  metals  by  the  usual  methods. 

Separation  of  the  Palladium  from  the  Platinum  Metals. — Considering  now 
the  ammonium-formate  precipitate,  Erdmann  and  Makowka1  have 
obtained  satisfactory  separations  of  palladium  from  platinum  and  iridium 
by  treating  the  solution  of  the  mixed  chlorides  with  acetylene.  Palladium 
comes  down  as  acetylide  and  the  other  metals  are  unprecipitated.  I  found 
also  that  rhodium  solutions  even  on  heating  were  not  precipitated  by 
acetylene.  As  for  osmium,  the  ease  with  which  it  oxidizes  and  the  high 
volatility  of  its  oxide  makes  its  elimination,  in  the  process  of  preparing 
the  palladium,  fairly  certain.  Ruthenium,  the  rarest  element  among  the 
platinum  metals,  need  hardly  be  looked  for;  still  it  was  sought  for  in  the 
iridium  found.  The  acetylene  method  was  used,  for  lack  of  a  safer  one, 
though  very  tedious.  In  solutions  at  all  concentrated,  I  find  the  palladium 
ceases  to  precipitate  long  before  it  is  entirely  removed  from  solution. 
Perhaps  this  is  due  to  the  accumulation  of  acid  liberated  in  the  process. 
At  least,  when  the  solution  is  separated  from  the  acetylide,  evaporated 
and  diluted  again,  acetylene  brings  down  another  portion.  After  five  or 
six  operations,  a  residual  solution  was  obtained  on  which  acetylene  had 
no  further  action.  The  acetylide  was  now  carefully  ignited  with  a  little 
ammonium  nitrate,  the  metal  redissolved,  and  the  whole  process  repeated. 
The  residual  solution  was  then  added  to  the  first  and  from  it  NH4C1  brought 
down  platinum. 

In  the  chlor-platinate  no  iridium  was  found.  It  was  ignited,  and  the 
metal  was  entirely  soluble  in  a  few  drops  of  aqua  regia.  It  was  again  pre- 
cipitated with  NH4C1  and  finally  weighed  as  platinum.  Pt=i.6  mg.  = 
0.007  Per  cent.  No  rhodium  was  found  in  the  filtrate.  In  the  attempt  to 
dissolve  in  aqua  regia  the  several  portions  of  metal  formed  by  igniting  the 
acetylide,  tiny  insoluble  residues  accumulated.  These  were  fused  with 
KHSO4,  which,  as  is  well  known,  dissolves  palladium  and  rhodium,  but  not 
iridium  or  platinum  if  the  temperature  is  kept  low.  The  soluble  portion 
was  dissolved  in  water  and  precipitated  with  ammonium  formate.  It  turned 
out  to  be  palladium,  since  it  was  precipitated  by  potassium  iodide  and  no 
trace  of  rhodium  was  found. 

The  portion  insoluble  inKHSO4  was  freed  from  silica  (which  came  from 
the  dish)  by  HC1  +  HF,  and  was  then  ignited  and  weighed.  Ir  +  Ru  (?)  = 
1.9  mg.  =0.008  per  cent.  When  fused  with  K3CO3  +  KNOV  some  blue  in- 
soluble IrO3  was  formed,  but  the  fusion  showed  no  yellow  color,  and  in  view 
of  the  minute  quantity  of  material,  it  was  not  thought  worth  while  to 
search  more  carefully  for  ruthenium. 


"Zeitschr.  anal.  Chetnie,  46,  145-150,  1907. 


96 


HIGH  TEMPERATURE  GAS  THERMOMETRY. 


The  final  precipitate  of  palladium  acetylide  was  changed  to  chloride, 
diluted,  and  saturated  with  SO2  for  gold,  but  none  appeared. 

Nothing  else  was  found  in  the  metal  except  a  trace  of  copper.  The  iron 
found  earlier  had  to  be  reprecipitated  several  times  from  chloride  solution 
by  ammonia  to  get  rid  of  palladium.  The  precipitate  was  finally  trans- 
formed into  sulphate  and  determined  volumetrically.  Fe=2.6  mg.  =  o.oio 
per  cent. 


ANALYSIS  OF  PALLADIUM. 


Au 

Ru 

Rh 

Pt 

Ir 

Cu 

Zn 

Fe.  . 


None 

None 

None 

.007 

.008 

Trace 

Doubtful  trace 

.010 

0.025  per  cent. 


In  the  following  table,  the  results  of  these  analyses  of  metals  for  the 
temperature  scale  are  summarized: 


SUMMARIZED  ANALYSES  OF  METALS.' 


Impurities 
stated  in  frac- 

Palla- 

Cobalt. 

Nickel. 

Copper.      Silver.      A^™1     Antimony.      Zinc.     Cadmium. 

tions  of  I  p.  ct. 

i                  1                  i 

Pt  

O.OO7 

none 

.0023 

'  .  OO  1  1        .  OOO  I 

Ir  

.008 

Rh  

none 



RU  

none 

Au  

none 

none 

none 

none      .0005 

Se  

none    

Te 

none 

AS.'.':::::: 



none 

none 

none      none      none      none        none      none 

Sb  

none 

none 

none      none      none    002 

Sn  

none 

none 

none      none    02(?)    | 

Hg  

.......    none    none     

Ag  

none 

none 

.  0006    none       none 

Pb  

none 

.0176 

.0001 

none      .0008    trace?      .051        .0860 

Bi  

none 

none 

none 

none      none    none       none 

Cu  

trace 

.0178 

.0523 

'  trace      .  003         .  004       none      trace 

Cd  

none 

none 

none 

none      none    none       .004 

Ni  

none 

none 

none      none      none       none       none     none 

Co  

none 

.1063 

none      none      none       none       none      none 

Pe  

.010 

.0006 

.0042 

.0038     .0011      .043         .007       .006       .0025 

Zn  

trace? 

none? 

none? 

.  0007     none  ?     none       none     trace 

Mn  

none 

none 

'  none     

Si  

none    190    ;  none 

C  

.  0003      .013     j  j  

s 

.013 

none 

.0020  '  .0004     .002    '   trace?  i  none  j  .0005 

P..  . 

r  none 

Ca  

none 

Na  

'  none 

K  

'  none 

Total  .  .  . 

.025 

.049 

.165 

.008        .003        .251     i   .031          .063     j   .089 

'A  blank  opposite  any  impurity  means  that  it  was  not  looked  for.        -Means  platinum  metals. 


THE    FIXED   POINTS.  97 

16.  THE  FIXED  POINTS. 

FURTHER  DETAILS  ON  THE  SUBSTANCES  EMPLOYED  FOR  THE    TEMPERATURE 

CONSTANTS. 

Zinc. — Two  samples  of  "C.  P.  sticks"  were  used,  both  from  Eimer  and 
Amend.  No  appreciable  difference  could  be  observed  between  their  melting- 
points.  Both  melting  and  freezing  points  were  sharp  and  measurable  to  a 
fraction  of  a  microvolt.  Successive  readings  did  not  differ  by  more  than 
one  microvolt.  The  charge  was  about  200  grams. 

Antimony.— Two  samples  of  metal  were  used,  both  from  Kahlbaum, 
and  no  appreciable  difference  was  found  between  their  melting-points. 
The  charge  weighed  about  150  grams.  The  melting-point  is  sharp  and  does 
not  differ  from  the  freezing-point  by  more  than  one  microvolt,  provided  the 
undercooling  which  always  precedes  solidification  does  not  exceed  15°.  If 
the  metal  is  undercooled  too  far  to  give  an  accurate  freezing-point,  the  fact 
is  easily  recognized  by  observing  that  the  thermo-element  does  not  return 
to  a  sustained  constant  temperature,  but  merely  rises  to  a  maximum,  then 
falls  again.  The  amount  of  undercooling  is  greater  the  higher  the  metal 
has  been  heated  above  its  melting-point  after  the  melting  is  complete. 

Silver. — The  charge  weighed  about  260  gms.  Only  one  supply  was  used. 
The  melting  and  freezing  points  were  sharp  and  agreed  within  one  microvolt. 

Gold. — A  new  charge  of  gold  was  used,  weighing  350  grams.  This  was 
obtained  from  Dr.  Eckfeldt  of  the  Philadelphia  Mint. 

Copper. — The  copper  was  obtained  in  the  form  known  as  "copper  drops 
cooled  in  hydrogen  "  (Eimer  and  Amend).  Only  one  supply  was  used.  The 
melting  and  freezing  points  were  not  quite  as  sharp  as  was  the  case  with 
silver,  but  always  agreed  within  i  microvolt.  The  temperature  is  very  sus- 
ceptible to  a  trace  of  oxide,  which  not  only  lowers  the  temperature  appreci- 
ably, but  makes  it  more  uncertain,  so  that  if  a  little  oxidation  has  taken 
place  it  is  recognizable  at  once.  Waidner  and  Burgess1  found  that  the  best 
commercial  electrolytic  copper  showed  an  average  difference  of  0.2°  in  the 
melting-point  from  the  purified  copper  drops.  Charge,  about  210  grams. 

Diopside  (Magnesium-calcium  melasilicate,  MgSi03  .  CaSiO3). — Two 
samples  of  chemically  pure,  artificial  diopside  were  used,  one  from  the 
preparation  of  Allen  and  White2  and  the  other  made  up  in  1909  by  G.  A. 
Rankin.  No  appreciable  difference  was  found  between  the  melting-points. 
No  freezing-point  can  be  obtained,  as  the  mineral  undercools  considerably. 
The  charge  used  was  3  grams. 

Xickel. — A  sample  of  specially  purified  electrolytic  nickel  was  obtained 
from  Kahlbaum.  The  analysis  showed  less  than  0.2  per  cent  total  impuri- 
ties. Care  must  be  taken  in  the  case  of  nickel  that  no  oxide  forms,  as  a 
fairly  sharp  break  can  be  observed  about  10°  below  the  melting-point, 
which  may  represent  the  eutectic  of  nickel  and  nickel  oxide.  This  break 
disappeared  when  the  nitrogen  was  replaced  for  a  few  minutes  by  hydrogen. 
This  lower  point  might  easily  be  mistaken  for  the  melting-point  of  the 
metal,  and  this  mistake  may  possibly  have  occurred  in  several  of  the  pub- 
lished determinations  of  the  melting-point  of  nickel.  Nickel  absorbs 
hydrogen  and  possibly  also  nitrogen,  and  after  cooling  frequently  showed 
excrescences  and  signs  of  "spitting"  such  as  occur  with  silver  in  air. 

•Phys.  Rev.  28,  p.  469,  1909.     Bull   Bur.  Stds.,  6,  p.  174.  1909  'Am.  Jour.  Sci.,  (4),  27,  1-4?-  1909. 


98  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

Cobalt.— Kahlbaum's  purest  cobalt  was  used,  containing  less  than  0.05 
per  cent  total  impurity.  It  was  in  the  form  of  fine  black  powder,  which  was 
compressed  into  blocks  for  convenience  in  handling.  The  results  obtained 
were  not  quite  as  satisfactory  as  with  nickel  on  account  of  the  higher  tem- 
perature and  more  rapid  contamination  of  the  thermo-element.  The  absorp- 
tion of  gases  seemed  to  be  less  than  was  the  case  with  nickel. 

Samples  of  Eimer  and  Amend's  "  98  to  99  per  cent  pure  "  nickel  and  cobalt 
were  also  tried.  The  difference  between  the  two  samples  of  nickel  was  not 
greater  than  the  uncertainty  in  the  melting-point  caused  by  contamination 
of  the  thermo-element.  The  "  98  to  99  per  cent  pure"  cobalt  melted  about 
3.5°  lower  than  the  pure  sample.  Since  the  impurities  in  nickel  are  usually 
chiefly  iron  and  cobalt,  and  those  of  cobalt  are  chiefly  iron  and  nickel,  and 
since  the  melting-points  of  all  three  are  close  together,  the  melting-points 
of  the  slightly  impure  metals  can  not  be  expected  to  lie  far  from  those  of  the 
pure  metals. 

Anorthite  {Aluminum-calcium  silicate,  CaAl2  Si?0x,  or  Al2Si05.CaSi03). — 
The  anorthite  used  was  made  from  pure  analyzed  materials  by  G.  A.  Rankin 
in  1909.  The  charge  was  about  3  grams.  The  melting-point  is  not  quite  as 
sharp  as  that  of  diopside.  Only  the  melting-point  can  be  obtained,  as  the 
mineral  undercools  considerably ;  it  may  even  cool  to  glass  without  crystal- 
lization, in  which  case  of  course  no  melting-point  will  be  obtained  on  the 
following  heating. 

Palladium. — About  350  grams  of  pure  palladium,  in  the  form  of  sheet, 
was  loaned  to  us  by  Dr.  Heraeus.  It  melts  and  freezes  quite  sharply,  mak- 
ing an  excellent  substance  for  a  fixed  thermometric  point.  The  greatest 
uncertainty  is  caused  by  the  vaporization  of  the  metal  and  consequent  con- 
tamination of  the  thermo-element  wire.  The  charges  used  weighed  128  and 
210  grams  respectively. 

Cadmium  and  Aluminum. — In  addition  to  the  fixed  points  just  described, 
two  other  metal  melting-points,  cadmium  and  aluminum,  were  incidentally 
determined.  Only  one  measurement  of  the  cadmium  point  was  made  on 
the  gas  thermometer,  and  this  chiefly  for  the  purpose  of  checking  the  extra- 
polation below  the  zinc  point.  The  conditions  of  melting  were  the  same  as 
for  zinc.  The  charge  weighed  215  grams. 

A  sample  of  pure  aluminum  obtained  from  the  Aluminum  Company  of 
America  was  melted  in  a  graphite  crucible  of  the  usual  size  in  an  atmosphere 
of  carbon  monoxide.  On  account  of  the  sensitiveness  of  aluminum  to 
silicon  contamination,  the  tube  carrying  the  thermo-element  was  provided 
with  a  thin  protecting  cover  of  graphite,  so  that  the  metal  came  in  contact 
only  with  pure  graphite.  The  freezing-point  was  sharp  and  constant.  The 
melting-point  was  less  sharp,  but  lay  within  0.5°  of  the  freezing-point. 

MELTING-POINT  MEASUREMENTS. 

Table  XIV  contains  in  summarized  form  the  readings  of  the  various 
thermo-elements  at  the  melting-points  of  the  standard  substances.  The 
values  are  in  microvolts,  on  the  basis:  Clark  cell  at  15°=  1.4328  volts. 
Each  value  given  represents  from  one  to  six  determinations  of  the  melting 
and  freezing  points  (in  the  case  of  aluminum,  diopside,  and  anorthite,  melt- 
ing-points only).  The  thermo-element  readings  are  given  at  the  constant 
temperatures  chosen  to  be  the  reference  points  of  the  nitrogen  scale.  Each 


THE   FIXED   POINTS. 


99 


element  is  represented  by  a  letter  (or  a  number  in  parenthesis).  Thus, 
element  C,  after  comparison  with  the  gas  thermometer  in  Table  XIII,  was 
used  to  determine  the  fixed  points  in  Table  XIV,  after  which  it  was  returned 
to  the  gas-thermometer  furnace  for  further  comparison.  "A"  in  copper 
read  10505  in  Dec.  (1908),  10502  in  Jan.  (1909),  10499  in  Feb.,  10504  in 
March,  10503  in  May,  and  10503  in  June. 

TABLE  XIV. — THERMO-ELEMENT  READINGS  AT  MELTING-POINTS. 


Date.           Zinc. 

Antimony. 

Silver. 

Gold. 

Copper.    Diopsidc. 

1908 

March  S  3408 

S  9056 

vS  10476 

V  34«' 

V  9050 

V  10477 

W  3406 

W  9057 

X  10485  ' 

X  3406 

X9o58 

Y  ,0577 

Y3437 

Y9,47 

Z  10438  i 

Z  3382 

Z  9019 

V  10478  i 

(42)  9066 

I 

(32)908. 

April  

X  5501 

W  10478 

W  5499 

X  10488 

t 

S  5503 

(42)  10489 

X  5505 

(31)10529 

j 

Y5545 

Z  5466 

June 

A  10500 

December  S  3418 

S  5506 

S  9069 

S  10481 

X34ii 

X5504 

X907I 

x  10484  ; 

Y  3437 

Y5547 

Y9'5' 

Y  10581 

A  3412 

A  5507 

A  9087 

A  10505 

(31)3426 

! 

(42)3406 

i 

1909 

T3.nu3.ry 

1  Y  10573  • 

I 

I  Z  10426 

1 

i  A  10502 

C  10458 

February  Y  3434 

Y  554' 

Y9'4> 

Z  10193 

Y  10573  ; 

Z  3382 

Z  5459 

Z  9022 

A  10260 

Z  10432 

A  3409  5 

A  5501 

A  9080 

C  10226 

A  10499 

03404 

C5489 

09049 

D  10231 

C  10460 

03403 

D;5488 

09055 

D  10467 

March          A  3413  5 

Z  5462  .  5 

Z  9019 

Z  10195 

Z  10432  ! 

C  3409.5 

A  5505.5 

A  9085 

A  10266 

i  A  10504 

D  3409 

Q5495 

0(9057 

C  10233 

c  10469  ; 

D.5495-5 

0^9059 

D  10235 

D  10473 

! 

C  10470  ; 

D  10475  i 

May  

Y  10571 

Z  10433  ' 

! 

A  10503  ' 

D  10469 

E  10534 

F  10533 

G  10531 

C,  i  0454 

June  

Z  546. 

¥9137 

Z  10195 

Y  10568   E  14230 

A  5504 

Z  9018 

A  10263 

Z  10432   E  14226 

E  5530 

A  9082 

D  10234 

A  10503   F  14229 

F  5530 

C9057 

E  10295 

D  10470   G  14229 

G  5529.5 

Egi'3 

F  10296 

E  10534   H  14231 

F9ii3 

G  10294 

F  10534   A  14200 

G  91  1  1 

G  10533 

HIGH   TEMPERATURE   GAS   THERMOMETRY. 


TABLE  XIV. — CONTINUED. 


Nickel. 

Cobalt. 

Palladium                       Anorthite. 

Miscellaneous. 

(Apr.  1909) 

(Sept.  1909) 

(Nov.  1  909)         (Nov.  1  909) 

Cd  (Mar.  1909) 

A  14947 

E  15404' 

H  16145             E   16144 

Z      2465 

D  14883 

E  15387' 

J   16144             E   16151 

C     2488 

Z  14850 

(Oct.igog)           J    16158              H   16145 

D     2486 

A  14943 

E  15391"            J    16151              F    16141 

Cd  (Mar.  1910) 

D  14881 

E  15439            B  16143             G   16148 

E    2502 

Z  14847 

F  15435             F  '6138    !         C    16060 

Z  14853 

G  15441             G  16145    : 

Al    (Mar.  1908) 

E  14974 

H  15436             C  16058 

Z    5758 

F  14975 

J    15445     ,      (Jan.  1910) 

Z     5757 

G  14975 
H  14977 

C  15359             J    '6150 
A  15409              J    16140 

V    5793 
Y    5836 

(Sept.  1909) 
E  14980 

(Apr.  IQII) 
L  16145 

NiO-Ni  (Apr.  1908) 

F  14981 

(Wire  method) 

Z  14723 

G  14981 

S  14750 

H.4983 

S  14743 

V  14747 

NiO-Ni  (Apr.  1909) 

Z  14712 

Z  14717 

'Commercial  metal. 


TEMPERATURE  OF  THE  FIXED  POINTS. 

Table  XV  contains  the  final  temperature  of  each  thermometric  point 
studied.  In  the  first  column  is  the  number  of  the  experiment  corresponding 
to  that  in  Table  XIII.  In  the  second  column  is  the  correction  in  degrees 
to  be  applied  to  each  of  the  thermo-element  readings  outside  of  the  bulb, 
integrated  from  the  readings  of  the  auxiliary  elements  as  described  on  page 
66;  in  the  third  column  is  given  the  corresponding  correction  in  microvolts. 
In  the  fourth  column  are  the  readings  of  the  standard  elements  on  the  out- 
side of  the  bulb,  corrected  as  above  mentioned.  In  the  fifth  column  are  the 
readings  of  the  same  thermo-elementsatthe  fixed  point  in  question,  as  ob- 
tained in  the  melting  or  freezing  of  metal  or  salt;  these  figures  usually 
represent  the  mean  of  a  considerable  number  of  determinations. 

In  the  sixth  and  seventh  columns  are  the  corresponding  figures  for  the 
element  inside  of  the  bulb.  In  this  case,  however,  no  correction  has  been 
applied  to  the  reading  of  the  element,  since,  being  located  practically  at  the 
center  of  the  bulb,  it  might  be  expected  to  represent  the  mean  temperature 
of  the  entire  volume  of  the  bulb. 

In  the  eighth  and  ninth  columns  are  the  temperatures  of  the  fixed  points 
derived  from  the  preceding  four  columns.  In  the  last  column  is  given  the 
weight  assigned  to  each  measurement.  In  assigning  these  weights  the  num- 
ber of  standard  thermo-elements  used,  the  amount  of  variation  in  p0,  and 
other  incidental  variables  were  taken  into  consideration. 

As  has  been  pointed  out  on  page  65,  the  relative  weights  to  be  assigned 
to  the  inside  and  outside  elements  are  different  at  different  temperatures: 


THE    FIXED   POINTS. 


(i)  on  account  of  the  difference  in  contamination,  and  (2)  on  account  of  the 
fact  that  the  inside  element  is  subject  to  the  influence  of  conduction  and 
radiation  from  below.  The  weights  assigned  were  as  follows : 


Temperatures. 


4OO-noo          3  i 

1100-1300°        2  i 

1300-1550°         i  i 


The  final  weighted  mean  of  the  inside  and  outside  elements  is  given  at 
the  head  of  each  section  of  the  table. 

In  the  last  section  of  the  table  are  given  various  points  which  were  deter- 
mined to  aid  in  interpolating  between  the  fixed  points  by  means  of  the 
thermo-element. l 

The  only  comment  which  need  be  made  here  on  the  data  in  Table  XV 
concerns  the  figures  given  under  the  heading  "copper  point. "  In  this  sec- 
tion of  the  table,  the  values  derived  at  the  two  different  initial  pressures 
of  gas  in  the  gas  thermometer  (217-221  mm.  and  346-347  mm.)  are  quoted 
separately  in  order  to  bring  out  the  fact  that  the  difference  between  the 
temperatures  obtained  from  these  two  pressures  is  less  than  the  experi- 
mental error.  In  the  other  sections  of  the  table  the  data  obtained  at  the 
two  pressures  are  not  separately  arranged.  Above  the  copper  point  only 
the  low  pressure  was  used,  as  the  high  pressure  would  have  exceeded  the 
range  of  the  manometer. 


'Since  the  completion  of  the  gas-thermometer  work,  Dr.  F.  M.  Jaeger  has  suggested  to  us  that  the  melting- 
point  of  lithium  silicate,  which  lies  at  1201°,  in  the  gap  between  copper  (1082.6°)  and  diopside  (1391"). 
would  be  a  good  calibration  point  for  thermo-elements.  He  made  and  analyzed  in  this  laboratory  a  pure 
sample  of  LijSiO:!,  and  determined  its  melting-point  with  one  of  the  thermo-elements  used  with  the  gas 
thermometer,  finding  the  value  1201.8°.  (Not  yet  published.)  The  composition  of  the  sample  was: 

Found.     Theoretical. 

SiO2 66.60  66.87 

Li,0 32.80  33.13 

Na..0 0.51 

K.,O None 

FeO None 

Fe.,0:, 0.016 

CaO 0.034 

We  measured  the  melting-point  of  the  same  sample  with  another  element,  finding  1200.6°.  Another 
sample,  prepared  by  Dr.  Crenshaw  in  this  laboratory,  gave  1199.6°.  Its  composition  was: 


SiO2.. 
Li.O.. 
FeO .  . 
H.,0.. 


65.89 
32-83 
0.05 


The  first  sample  is  slightly  deficient  in  Li._.O,  the  second  is  slightly  d 

taken  as  fairly  representative  of  the  average  sample  obtained  by  synth 

reproducibility  of  the  point  is  probably  about  i°. 

' 


ual  determinations  are  as  follows: 

Prepared  by  — 

Observed  by  — 

Element 

aeger 
aeger 

Jaeger 
Jaeger 

C 
C 

aeger 
aeger 

Jaeger 
Jaeger 

C 
C 

aeger 
aeger 
aeger 

Jaeger 
Jaeger 
Sosman 

C 
C 
G 

Jaeger 

vSosman 

G 

Crenshaw 

Sosman 

H 

Crenshaw 

Sosman 

H 

Crenshaw 

Sosman 

G 

Crenshaw 

Sosman 

G 

201  .8 

201  .2 

202  .0 
202  .0 
201.8 
20O.6 
2OO.6 
199.2 
199.8 
199.7 
199.8 


Mean     1 200 .  o° 


ficient  in  SiO.,,  and  both  may  be 
sis  from  I,i,C6:i  and  SiO,.     The 


HIGH  TEMPERATURE   GAS  THERMOMETRY. 


The  significance  of  the  comparison  between  the  inside-wound  and  out- 
side-wound furnaces,  which  appears  in  the  first  half  of  the  section  on  the 
copper  point,  has  been  commented  on  elsewhere  (see  p.  56). 

TABLE  XV. — TEMPERATURES  OF  THE  FIXED  POINTS. 


Integrated  cor- 

Standard elements. 

Temperature. 

rection  to  outside 

1 

Exp. 
No. 

elements. 

Outside 
corrected. 

Fixed             Inside            Fixed 
point.        uncorrected.       point. 

outside 
element. 

By       jWei*ht. 
inside 
element.  ' 

Degrees 

M.  V. 

Zinc  Point.    418.2° 

22 

O.O°        O 

A  34'4          34H 

Y3436 

3436 

4-8.1 

>,      4.8.4°         2 

23 

-0.3 

-3 

A  3405 

3410.5       Y3425 

3435 

418.0 

418.4 

2 

3i 

o.o        o 

A  3410 

3410.5       Y  3436 

3435       418.3 

1      418.2 

2 

32 

-0.2 

—  2 

A  3402 

3410 

Y3425 

3434 

418.0 

!  418.1 

2 

50 

—  O.  I    !    —  O.  5 

A  3413 

34'  I 

4l8   2 

D  3405  .  5 

2t'  " 
3406 

Z3384.5 

3382 

T 
418.4 

418.3 

;  418.2 

3 

73 

—  O.2 

—  2 

A  3401 

3413-5 

418.4 

E  3417 

342Q 

4l8.3 

F34I2 

•*.\~  i        .  ,  .  f  
3429        

418.8 

* 

03414 

3429 

z  3370   i   3382 

418.6 

418.5 

i    418.3       4 

| 

•;•__.     

Weighted  mean, 

418.  2C 

IH 

Antimony  Point.    629.2° 

24;  -o.i      -i          A  5509    ;     5503          Y  5550         5546       629.2° 

629.4°!        2 

25 

-0.5      -5           A  5496         5503          ¥5529         5545       629.0 

629  .  9         2 

33 

-0.  I 

—  I 

A  55'3         5503          Y  5553          5544 

629.2 

629.3 

2 

34 

-0.5 

-5 

A  5505         5503          Y  5537 

5543 

629.  I 

629.9 

2 

52 

-0.5 

—  5 

A  5505         5503 

629.6 

D  5490 

5492     j      Z  5463         5460 

629.2 

629  .~4 

629.  I 

2 

•JA 

—  0.  2 

—  2 

A  55  14 

5504 

628.2 

/^T 

E5533 

JJ^"r 
5530 

628.8 

F  5526 

553O 

629.  5 

j.     77-AVJ 

05527 

jjj^* 
5530 

Z546. 

5461 

629.4 

629.0 

629.1 

4 

87 

-0.3 

-3 

F55'7        5530     

628.9 

E  55'7        5530     

628.9 

. 

A  5481     i     5504    '  

629.9 

G55'3 

5530 

Z  5437         5461 

629.3 

629.2 

629.9        4 

Weighted  mean, 

629.1° 

629.  5  j 

THE   FIXED   POINTS. 

TABLE  XV.— TEMPERATURES  OF  THE  FIXED  POINTS — Continued. 
Silver  Point.     960.0° 


103 


Integrated  cor- 
rection to  outside 
Exp.            elements. 

Xo-                                           rim  M 

Standard  elements. 

Temperature. 

Fixed 
point. 

9057 
9083 
9083 
9082 
9082 
908, 
9081 

9084 
9058 

9085 
9058 

9082 
9113 
9113 
911  1 

9113 
9113 
9082 
91  1  I 

9"3 
9H3 
9082 
9III 

j 

Inside 
uncorrected. 

X  9IOO 
Y9n9 

Y9I42 

Y9.63 
Y9,56 
Y9.3, 

Z  9010 

Fixed 
point. 

9071 
9141 
9141 

9141 
9141 
9141 

9019 

By 
outside 
element. 

959  4° 
959-9  ! 
959  9  i 
959-7 
959  4 
959  6 
959  3 

960.7 

96  1  .  2 

Weight. 
By 
inside 
element. 

958.0°          I 
958.6           2 
960.4           2 

959  4         ' 

958.7      I 

958.5           2 

959  7         2 

961.1         2 

j 

960.3           2 

961.2         4 
961.5         4 
960.1         4 

Degrees.    M.  V.        corrected. 

6     +0.8     +9         W  9070 
26     —0.3     -3          A  9087 
27      —0.8      —9          A  9066 
35      -0.7      -8          A  9079 
36      —o.i      —  i           A  9097 
41      -0.3      -3           A  9083 
42      -0.8     —9          A  9076 

63      -0.6     -7          A  9079 
09048 

j 

68     -0.7     -8          A  9080 
09051 

76       —0.2        —2              A  9088 

E  9112 
F9097 
09106 

81      +0.3      +3           F  9132 

A  9083 
09125 

i 

88     +0.3      +3           F9I43 

E9>39 
A  9093 
G9'35 

j 

960.9   : 
960.4 

Z9oi3 
Z  9002 
Z90I5 

9019 
9018 
!       90l8 

960.4 

959-2 
959  7 
961.2 
960.2 

960.1 

i    959-5 
959-6 
;     961  -  1 
i     960.0 

960.0 

959-0 

i     959-4 
!     960.7 
1     959-6 

'Z9o36' 
Weighted 

9018 

i 

mean, 

i     959-7 

959  9° 

960.2°  . 

64     —0.3     —3        A  10262 
D  10226 

69     -0.4      -4        A  10253 
D  10217 

77      -0.3      -3         A  10255 
E  10282 
F  10263 
G  10276 

Gold  Po 

10265 

10233 

10266 
10234 

10263 
10295 
10296 
10294 

int.     1062.4 
Z  10178 

Z  10169 
Z  10161 

0 

1062.4° 
1062.8 

1063.4°         2 

i 

1062.3           2 

| 

4 
1063.3 

10193 
10193 

10193 

1062.6 

1061.4 
1061.7 

1061  .6 

.    1061.2 
.    1061.6 
1063.4 
1062.  i 

1062.1 

104  HIGH    TEMPERATURE   GAS   THERMOMETRY. 

TABLE  XV. — TEMPERATURES  OF  THE  FIXED  POINTS — Continued. 
Gold  Point.     1062.4° — Continued. 


Exp. 
No. 

82 

Integrated  cor- 
rection to  outside 
elements. 

Degrees.    M.  V. 

+0.4     +4 

i 

' 

Standard  elements. 

Temperature.        ' 

Outside 
corrected. 

F  10303 
E  10304 

A  10256 
G  10296 

Fixed 
point. 

10296 
10295 
10263 
10294 

Inside 
uncorrected. 

•a?  ;  Jt  iS.  """ 

P01"1'       element.  ;  element. 
1061  9° 

Z  10181 
Weighted  rr 

.     1061.8 

i  -063.  « 
10193  1062.4 

J  1062.3      1063.6°       4 

lean,      i  1062.2°    1063.2°; 

Copper  Point.     1082.6°  (Lower  Pressure. 

Po  =2  17-22  1  mm.) 

2 

9 
ii 
18 
'9 

20 
28 
29 

11 

39 
43 

44 
45 

89 
126' 

+  1.2 
+  1.0 
+  1.2 

+0.7 

-0.4 

-0.8 
-0.3 
-0.9 
-0.6 
-0.3 
+0.7 
-0-3 
-0.9 
+0.6 

+0.5 
+0.4 

+  '4 

+  12 
+  '4 

+  8 
-  5 
-  9 

—  10 

-  7 

;j 

-  4 
—  10 

+  7 

+  6 

+  5 

W  ,0457 
W  10495 
W  .0487 
A  10510 
A  10501 
A  10488 
A  10512 
A  10494 
A  10504 
A  10509 
A  10517 
A  10512 
A  10501 
A  10515 

F  10546 
E  10544 
A  10496 
G  10538 

E  10631 
F  10627 
G  10621 
H  10623 

.0478 
10478 
10478 
10502 
10502 
10502 
10501 
10501 
10501 
10500 
10500 
10500 
10499 
10499 

10534 
10534 
10503 
'0533 

10534 
10534 
10533 
10535 

X  ,049, 
X  ,0555 
X  10512 
Y  10612 
Y  10584 
Y  ,0555 
Y  10593 
Y  10556 
Y  10576 
Y  10585 
Y  10617 
Y  10595 
Y  10568 
Y  10617 

Z  10428 
C  10567 

1081.7°!  !       i 
1082.2  i  i 
1083.1  1  i 

10573        IO82.2     2 

10573      1082.0      1081  .0°       3 
10573      1082.  i      1082.5         3 
10573      1082.1      1081.4         3 
10573      1082.2      1083.0    ,     3 
10573      1082.0      1082.0         2 
10573      1082.2      1081.9         2 

1  082  .3     2 

10573      1082.2      1081  .3         3 
10573      1082.0      1082.6         3 
i  082  .3    2 

!   1081.8 
i   1081.9 
1083.4 
10432      1082.4 

1082.4      1083.1         4 

1082.2 
1082.6 
1083.0 
10470      1083.0 

1082.7      1082.5         4 

Weighted  mean,       1082.2°    1082.2° 

Copper  Point  —  (Higher  Pressure.     p(, 

=  346-347  mm.) 

60 
65 
70 

-0.7 
-0.8 
-0.3 

-8 
-9 
~4 

A  10500 
D  10465 

A  10502 
D  10465 

A  10508 
D  10475 

10502 
10470 

10503 
10471 

10504 
10472 

1083   4° 

Z  10422 
Z  10420 

10432     1083.6 

1083.5       1084.1°         I 

i  083  .  o 

10432       1083.4 
1083.2       1083.9            2 

'<  1082  4 

Z  10444 

10432      1082.6 

1    1082.5        1081.8            2 

'Made  with  outside-wound  furnace.     See  page  56  and  Fig.  10. 


THE    FIXED   POINTS. 

TABLE  XV. — TEMPERATURES  OF  THE  FIXED  POINTS — Continued. 
Copper  Point.     (Higher  Pressure.     p(}  =  346-347  mm.) — -Continued. 


105 


Exp. 
No. 

Integrated  cor- 
rection to  outside 
elements. 

Outside 
Degrees.    M.  V.        M™*"1- 

Standard  elements. 

Fixed              Inside             Fixed 
point.         uncorrected.       point. 

Temperature. 

By                By         *««»'• 
outside    •     inside 
element.  :  element. 

78 

—  o.i        —  i       A  10502 
E  10528 

10503  
IO534 

1081.4° 

1081  8 

F  10509 
G  10522 

10534  
10533  Z  10404  10432 

1083.4 
1082.3 

83 

+o.  i       +i       F  10535 

10534  

1082.2      1083.7°;       4 
,   1082.  i 

E  10535 
A  10488 
G  10527 

10534  
10503  
10533  z  I04°3  10432 

1082.1 
1083.5 
1082.7 

84 

+0.7       +8       F  10544 
E  10542 

10534  

1O>34 

1082.6      1084.6         4 

i  082  .  o 
1082  3 

A  10493 
G  10533 

10503  

10533  Z  10426  10432 

1083.8                         4 
1083.0 

1082.8      1083.5 

Weighted  mean, 
Mean  of  2  pressures, 

1082.7°    1083.7° 
1082.5°    1082.9° 

D 

iopside  Point.     1391  .2° 

96 

o.o           o       E  14227 
F  14222 
G  14245 
H  14251 

14228  
14229  
14229  
14231  Z  14121  14103 

I392.l°i 
1392.5 
1390.7 
1390.4 

07 

_)_  |  o     -(-13      E  14260 

14228 

1391.4      1390.5°        I 
i  392  .  4 

F  14254 

14229  

i  393  .  o 

G  14287 
H  14295 

14229  
14231  Z  14156  14103 

99 

IOO 

—  o.  i      -   i      E  14213 
F  14195 
G  14215 
H  14212 

+0.7     +9       E  14251 
F  14244 
G  14268 
H  14273 

14228  
14229  
14229 

14231  Z  14099  14103 
14228  

14229  
14229  

14231  Z  14156  14103 

1391.4  i  1390.5  ;       i 

"394-5 
1396.0 

1394-4 

1394.8 

1394.9           1393.7     ;               I 

1394-4 
1395.0 
I393-I 
1392.9 

1393.8            1391.8                    I 

io6 


HIGH   TEMPERATURE    GAS   THERM OMETRY. 


TABLE  XV. — TEMPERATURES  OP  THE  FIXED  POINTS — Continued. 
Diopside  Point.      1391.2° — Continued. 


Integrated  cor- 
rection to  outside 
Exp.           elements. 
No. 

Standard  elements.                                     Temperature. 

Weight. 

Outside 
corrected. 

Fixed              Inside            Fixed 
point.        uncorrected.       point. 

By               By 
outside         inside 
clement,      element. 

Degrees.   M.  V. 

103     -0.4     -  5 

104      +i.o      +13 
106     -0.6     -  8 

107       +0.9       +12 

130     —0.6     —  7 

E  1421  i 
F  14204 
G  14217 
H  14230 

E  14242 
F  14212 
G  14249 
H  14262 

E  14228 
F  14225 
G  14233 
H  14243 

E  14248 
F  14237 
G  14245 
H  14252 

E  14243 
F  14241 
G  14249 
H  14239 

14228       !  

14229    !  

'4229       
14231         Z  14124        14103 

14228       
14229       
14229       

14231         Z  14155     1  14103 

14228    i  
14229    

14229                      .            .... 

1392.5° 
1393-1 
1392.1 
I39I.2 

3 
3 

2 
2 

3 

1392.2       1389.4° 

1392.5 
"394-9 
1392.0 
1391.1    : 

1392.6       1389.3 

I391-7   ! 
1392.0 
I391-3 
1390.7       1390.0 

14231         Z  14123       14103 

1 
14228     

14229    
14229    

14231         Z  14152     i   14103 
14228 

1391.4 

1391.9 
1392.8 
1392.2 
1391.8       1389.4 

1392.2 

1390.2 
1390.6 
1389.9 
1390.6 

I390-3        1392.0 

14230 

14230 

14228        C  14146       14153 
Weighted  mean, 

1392.00     I390.40 

Nickel  Point.     1452.3° 

109     +0.7     +  8 

III             0                    O 
112        +0.9       +11 

E  15028 
H  15027 

E  14980 
H  14978 

E  14971 
F  .4958 
H  14991 

M977     i  
14980         Z  14903       14850 

14977     
14980         Z  14867        14850 

14980       z  14872   ;  14850 

I451.20 
I45I.6 

2 

1451.4      '45'-'° 

1453-2 
'453-7 

1453.5      1452.1 

1453-8 
H54-9 
1452.4 

1453-7      145I-5 

THE    FIXED   POINTS. 

TABLE  XV. — TEMPERATURES  OF  THE  FIXED  POINTS — Continued. 
Nickel  Point.     1452.3° — Continued. 


107 


Integrated  cor- 
rection to  outside 
Exp.            elements. 
Xo- 

Standard  elements. 

Fixed              Inside             Fixed 
point.         uncorrected.        point. 

'4977      
14978         A  14982        14945 

'4977      
14978        A  14996       14945 

'4977      
14976      
14981      
14977        C  14882        14898 

Weighted  mean, 

Temperature. 

By                By          Weight, 
outside         inside 
element,      element. 

'454-3° 

'453  9 

Degrees.    M.  V.        corrected. 

123     —0.6     -  7      E  14984 
F  14989 

124     -fo.8     +10      E  14989 
F  14994 

133     -0.3     -  4      E  14954 
F  14958 
G  14955 
H  14946 

1454.1      14518°       2 

1454.6 
1454-3 

1454.5      1451    5          ' 

1451.9 

145L5 

1452.2 
1452.5 

1452.0      1451.3         4 

1453.0°    1451.6° 

Cobalt  Point.      1489.8° 

115     +0.1     +  i      E  15390      15439     
F  15375        "5435         A  15357 

116     +  1.4     +17       E  15428        15439      
F  15434        '5435      
G  15435        15441         A  15421 

118     —0.5     —6      £15385       15439     
F  15383       15435      
G  15393       15441         A  15382 

120     —0.4     —   5       E  15381        15439      •••• 
F  1537'        '5435      
G  15363        15441         A  15379 

121      +0.7     +9       E  15406       15439      
F  15405        15435 
G  15398       15441         A  15412 

Weighted 
Palladium  Point.     1549. 

134     -0.7      -9       E  16151        16143     1  
F  16161        16138 
G  16139       '6145     i  
H  16147       '6145     j    C  16075 

| 

15409 
15409 
15409 
15409 

15409 
mean, 

2° 

'.'.'.'.'.'. 
16058 

1488.7° 
.489.6 

.489.1      .488.9°       3 

1490.5 
1489.7 
1490.1 

1490.  i      1488.6        3 

1491.7 
1491.6 
'491-3 

.491.5        1489.6            2 

1491.7 
1492.1 
'493-3 

1492.7        14894             1 

1492.0 
1491.8 
1492.8 

1492.2        1489.1             1 

1490.6°     1489.0° 

!  1549.5°' 
.548.3 

!  1550.6 

!   1550.1 

j   1549.6°    1548.8° 

io8 


HIGH    TEMPERATURE    GAS   THERMOMETRY. 


TABLE  XV. — TEMPERATURES  OF  THE  FIXED  POINTS — Continued. 
Anorthite  Point.     1 549.  5°. 


Integrated 
rection  to  ou 
Exp.            clement 
No.    ! 

Degrees.    Jk 

cor- 
tside 

Standard  elements. 

Temperature. 

1. 

Outside 
1   y        corrected. 

Fixed 
point. 

Inside             Fixed 
uncorrected.       point. 

By         W"ght 
outside         inside 
element,      element. 

'34  ;  -0.7     ' 

i 
i 

-9       E  16151 
F  16161 
G  16139 
H  16147 

(6148 
16141 
16148 
16145 

C  16075       16060 

1549-9° 
.548-6 
1550.9 
1550.0 

•549-9°    «549  o° 

Interpolation  Points. 

49     -o.i 

-  1           A  2486 
D  2482 

2492 
2486 

Z  2462         2465 

320.  2  °j 
320.0 

320.1°     319.9° 

Mean  for  cadmium, 

320.0° 

51         o.o 

o          A445I 
D4439 

I   . 

4450 
4443 

Z4413         4117 

524.6  • 
525.0 

524.8°      525.1 

Mean  for  A  =  4450, 

5*4-9" 

62         o.o 

o          A  7895 
07869 

7900 
7881 

Z  7829         7848 

854.2 
854-9 

854.6°     855.5 

67        —  0  .  2 

-2          A  7883 

07859 

7900 
788l 

Z  7820         7848 

854-0 

854.5 

854.  3°i     855.0 

Mean  for  A  =  7900, 

854-7° 

128        —  O.2 

-2       E  12004 

F   I2OOI 

G  12008 

H    I2OOO 

I2OOO 
12OOI 
I2OOI 
I2OO3 

€11914        ii  928 

1206.3 
1206.7 
1206.  i 
1206.9 

1206.5°    1207.8 

132      +o.  i 

-f  i        E  11947 
F  11952 
G  11950 
H  11941 

12OOO 

11997 

I2OOI 

12003 

:::::::::::!::::::: 

C  1  1887       1  1928 

1206.0 
1205.3 
1205.8 
1206.8 

1206.0°    1205.0 

Mean  for  E  =  12000, 

1206.4° 

129      —  0.6 

| 

-5         E  13107 
F  13102 
i    G  131  10 
H  13101 

13100 
i  13101 
I  13101 

13103 

1297.4 
1297.9 
,297.2 
.298.1 

;  

C  13007       13023 

1297.7°    '299-3 

Mean  for  E  =  1  3  1  oo, 

1298.5° 

INTERPOLATION   BETWEEN   THE    FIXED   POINTS.  109 

17.  INTERPOLATION  BETWEEN  THE  FIXED  POINTS. 

The  preparation  of  formulae  to  represent  the  relation  between  the  tem- 
perature defined  by  the  gas  thermometer  and  the  electromotive  force  of  a 
thermo-element  has  always  been  a  cause  of  considerable  dissatisfaction, 
both  to  the  maker  and  the  user.  The  chief  reason  for  this  is  perhaps  the 
fact  that  the  formulae  used  have  been  applicable  only  to  limited  portions 
of  the  curve  and  have  therefore  given  no  suggestion  of  physical  significance. 
In  the  Reichsanstalt  publication1  the  data  extended  from  300°  to  1 100°  and 
included  several  good  fixed  points  (melting-points  of  pure  metals)  between 
which  no  interpolation,  however  rough,  could  go  far  astray.  Accordingly, 
in  so  far  as  interpolation  was  concerned,  but  little  attention  required  to  be 
given  to  the  formulation  of  this  relation.  It  was  sufficient  that  a  simple 
formula  of  the  form 

e=  -a  +  bt+ct2 

could  be  made  to  represent  the  observations  between  300°  and  1 100°  within 
the  limits  of  errors  of  observation. 

If  the  investigator's  responsibility  could  be  made  to  end  with  the  repre- 
sentation of  his  own  observations,  no  serious  difficulty  would  arise,  but  such 
a  formula  when  published  is  placed  in  the  hands  of  many  who  do  not  realize 
that  no  physical  significance  was  attached  to  the  formula  by  its  author  and 
that  its  extrapolation  in  either  direction  would  be  fraught  with  grave 
danger.  A  mere  inspection  of  the  equation  is  sufficient  to  show  that  the 
electromotive  force  does  not  become  zero  for  zero  temperature,  thereby 
immediately  proving  that  extrapolation  downward  does  not  correspond  to 
the  observed  readings  of  the  thermo-element.  In  the  Reichsanstalt  equa- 
tion this  constant  term  was  in  fact  sufficiently  large  to  lead  to  absurdities 
if  the  extrapolation  was  continued  far  below  300°. 

Notwithstanding  the  warning  contained  in  this  situation,  extrapolation 
upward  of  the  thermo-electric  curve  has  been  employed  almost  universally 
for  the  determination  of  temperatures  above  1100°,  not  only  for  direct 
determinations  of  temperature  with  the  thermo-element  itself,  but  also  for 
the  calibration  of  optical  pyrometric  apparatus.  The  absence  of  absolute 
determinations  in  this  region  has  left  this  practice  in  undisturbed  security 
until  recently,  when  some  doubt  has  been  thrown  upon  the  validity  of  irre- 
sponsible upward  extrapolation  by  various  observations: 

1 i )  The  increase  in  the  accuracy  now  attainable  with  the  optical  pyrometer 
has  given  an  independent  thermal  scale  comparable  with  that  of  the  thermo- 
element and  overlapping  the  same  region.    The  two  curves  have  not  been 
found  to  correspond. 

(2)  Experimental  determinations  of  the  melting-point  of  platinum  by 
continuing  observations  of  the  thermo-element  up  to  a  point  where  a  por- 
tion of  its  platinum  wire  melts,  have  been  undertaken  in  the  national  labora- 
tories of  Germany,  England,  and  the  United  States,  and  have  yielded  a 
value  measured  upon  the  extrapolated  thermo-electric  curve  of  about  1710°. 
The  agreement  in  the  different  determinations  was  good  and  the  result 
found  general  acceptance  for  a  time.     More  recently,  as  has  been  stated, 

'Holborn  and  Day,  1900,  loc.  cit. 


HIGH   TEMPERATURE    GAS   THERMOMETRY. 


Holborn  and  Valentiner  have  made  successful  measurements  with  the  gas 
thermometer  at  the  temperature  of  melting  palladium,  and  although  high 
accuracy  was  not  attempted,  it  became  clear  that  the  palladium  point 
obtained  by  extrapolating  with  the  thermo-element  was  much  too  low  and 
by  inference  the  platinum  point  even  more  so,  for  the  various  optical  methods 
give  opportunity  for  a  very  good  determination  of  the  temperature  differ- 
ence between  the  melting-points  of  the  two  metals.  The  most  recent 
estimates  of  the  platinum  melting-point  obtained  in  this  way  have  placed  it 
between  1750°  and  1755°,  indicating  that  the  upward  extrapolation  with 
the  thermo-element  has  given  rise  to  an  error  of  about  45°  at  the  platinum 
point  (see  following  table.) 

The  data  obtained  in  the  present  investigation  throw  much  light  upon 
this  situation.  If  we  take  the  observations  of  our  series  over  the  range 
covered  by  the  Reichsanstalt  scale  (300°  to  1 100°)  and  write  an  equation  for 
these  of  the  same  type  as  that  used  at  the  Reichsanstalt,  it  will  read 

e=  —  302+8. 2356/+.ooi6393/2 

and  this  equation  will  reproduce  the  temperatures  of  the  standard  melting- 
points  which  fall  in  this  region  with  a  maximum  error  -of  3  microvolts 
(=  0.3°),  an  accuracy  far  within  the  errors  of  observation.  But  if  we  extra- 
polate this  curve  in  accordance  with  the  general  practice  above  described, 
and  compare  the  resulting  electromotive  forces  with  our  observations  be- 
tween 1100°  and  the  melting-point  of  platinum,  a  somewhat  startling 
surprise  awaits  us.  Although  the  curve  below  the  copper  point  is  a  prac- 
tically perfect  reproduction  of  the  observations,  it  diverges  from  the  gas- 
thermometer  scale  at  the  melting-point  of  palladium  by  248  microvolts, 
which  represents  a  temperature  error  of  20°.  At  the  platinum  melting- 
point  it  has  grown  to  45°.  This  comparison  is  made  in  the  table  below. 


Temp.      ;  Observed. 

Calculated. 

Observed- 
Calculated 

Observed- 
Calculated. 

mv. 

mv. 

mv. 

0 

Zinc  

418.2  ,      3429 

3429 

0 

0.0 

Antimony  

629.2  |       5530 

5530 

0 

0.0 

Silver  

960  .0        9113 

9115 

—  2 

—  0.2 

Gold  
Copper  

1062.4       10295 
1082.6       10534 

j 

10298 
10534 

O 

-0.3 
0.0 

J 

Zxlrapolatio 

1. 

1206.4          I2OOO 

12019 

-    IQ 

—   1.6 

1298.5          I3IOO 

13156 

-    56 

-  4-7 

Diopside  

1391.2          14228 

14328 

—  ,00 

-  8.3 

Nickel  

1452.3          14977 

,51  16 

-139 

-11.5 

Cobalt  

1489.8          15439 

15606 

-167 

-13.8 

Palladium  

1549.2          16143 

16391 

-248 

—20.  6 

Platinum  

(1752.  )*      ,86,6 

19,59 

-543 

-45-3 

If,  on  the  other  hand,  we  represent  /  as  a  function  of  e,  using  the  same 
data  as  before,  the  equation  will  take  the  form 

/=47. 2+0.1 1297*- 1.3946  (io)~V 


'Values  in  parenthesis  extrapolated  (see  p. 


INTERPOLATION   BETWEEN   THE   FIXED   POINTS.  1 1 1 

This  curve  passes  through  the  fixed  points  below  1 100°,  nearly  as  accurately 
as  the  previous  one,  and  is  also  quite  competent  to  interpolate  tempera- 
tures through  the  range  of  the  old  standard  scale.  Extrapolating  this  in 
turn  up  to  the  platinum  point  and  comparing  it  with  our  gas-thermometer 
measurements  in  the  higher  region  leads  to  temperatures  about  42°  too  low 
for  palladium  and  85°  too  low  for  platinum. 


Observed.    Calculated, 


Zinc 418.2  418.2  o.o 

Antimony 629.2  629.3  —  o.i 

Silver 960.0  960.9  —0.9 

Gold 1062.4  1062.4  ° 

Copper 1082.6  1082.5  +0.1 


Extrapolation. 


1206.4  I2O2.O  +4.4 

1298.5  1287.8  +10.7 

Diopside 1391 .2  1372.2  +19.0 

Nickel 1452.3  1426.3  +26.0 

Cobalt 1489.8  1458.9  +30.9 

Palladium '5492  1507.4  +41.8 

Platinum (1752.)  1667.  +85. 


The  untrustworthiness  of  the  present  practice  of  extending  thermo-ele- 
ment  values  obtained  below  1 100°  into  the  region  above  that  temperature 
is  therefore  abundantly  demonstrated. 

We  were  unable  to  find  a  simple  parabola  with  which  to  represent  the 
whole  series  of  observations  between  300°  and  1550°  within  the  errors  of 
observation.  The  simplest  procedure  is  therefore  to  divide  the  long  curve 
into  two  parts.  This  plan  is  carried  out  below  in  the  form  in  which  it  will 
probably  be  found  most  useful.  A  parabola  passing  through  zinc,  antimony 
and  copper  reproduces  the  results  over  that  temperature  range  within  the 
errors  of  observation.  A  similar  parabola  through  copper,  diopside,  and 
palladium  gives  the  upper  temperatures  as  accurately  as  they  were  meas- 
ured. These  two  equations  offer  a  means  of  safe  and  convenient  inter- 
polation throughout  the  entire  range  of  gas-thermometer  measurements.  In 
this  series  are  included  certain  gas-thermometer  measurements  given  at 
the  end  of  Table  XV,  which  were  made  at  temperatures  between  the  fixed 
melting-points,  for  the  purpose  of  checking  the  interpolation  formula,  to- 
gether with  a  single  gas-thermometer  determination  of  the  cadmium  melt- 
ing-point and  the  extrapolated  platinum  point  described  on  p.  115.  The 
temperature  854.1  appears  here  corrected  by  —0.6°,  since  the  series,  of 
which  this  measurement  formed  a  part,  showed  a  systematic  difference  of 
about  this  amount  from  the  final  average  of  antimony  and  silver,  which  lie 
on  either  side  of  this  point. 


HIGH  TEMPERATURE   GAS  THERMOMETRY. 


CADMIUM  TO  COPPER.     e=  —302+8. 2356^+. 0016393^ 


Tempera- 
ture. 

Observed.    Calculated. 

Observed-  Observed- 
Calculated     Calculated. 

Cadmium  
Zinc  

0 

320.0 
418.2 
524.9 

620.2 

854.1 

960.0 
1062.4 
1082.6 

mv. 
2502 
3429 
4470 
5530 
7929 
9113 
10295 
10534 

mv. 
2501 
3429 
4472 
5530 
7928 
9115 
10298 
10534 

mv. 
+  i 
o 

—  2 
O 

+  1 
—  2 

-3 

0 

+0.1 

o.o 

—  0.2 
O.O 
+0.1 
—  0.2 
-0.3 
O.O 

Antimony  

Silver  
Gold  

Copper  

COPPER  TO  PALLADIUM. 

e=  —  1941  +  1  i.i  746^+.  00032  i6its 

Copper  

Diopside  
Nickel  
Cobalt  
Palladium  
Platinum  

1082.6 
1206.4 

1298.5 
1391.2 
1452.3 
1489.8 
1549.2 
(1752.) 

10534 

12000 
I3IOO 
14228 
M977 

'5439 
16143 
18616 

10534 
12008 
13111 
14228 
14967 
15421 
16143 
18624 

0 

-  8 
-ii 

0 

+  '0       . 

+18 

o 

-  8 

0.0 

-0.7 

-09 
O.O 

+0.8 

+  '•5 

O.O 

-0.7 

It  is  possible  to  write  a  cubic  equation  which  will  reproduce  the  entire 
series  from  zinc  to  palladium  without  error  greater  than  the  normal  accuracy 
of  the  observations  themselves,  but  even  this  equation  goes  astray  at  the 
platinum  point  by  5°,  and  reminds  us  again  of  the  absence  of  physical  sig- 
nificance in  all  these  formulae.  The  equation  offered  makes  no  pretensions 
to  a  least-square  solution  with  balanced  residuals,  but  is  arranged  so  that 
the  greatest  uncertainties  are  found  in  that  portion  of  the  curve  where  the 
greatest  experimental  error  lies.  The  coefficients  were  rounded  off  for  con- 
venience of  computation. 

CADMIUM  TO  PALLADIUM.     e.  =  —169+7. 57'+o. 002648^ -0.0000004724*" 


;     Temp.        Observed.    Calculated. 


Observed-  Observed- 
Calculated.  Calculated. 


0        1 

mv. 

mv. 

mv. 

0 

Cadmium  

.  .  .      320.0 

2502 

2509 

—  i 

-0.8 

Zinc  

...     418.2 

3429 

3425 

+  4 

+0.4 

524.9 

4470 

4466 

+  4 

+0.4 

Antimony.  .  . 

629.2 

5530 

5525 

+   5 

+0.5 

854.1 

7929 

7934 

-    5 

-0.5 

Silver  

.  .  .     960.0 

9113 

9121 

-  8 

—0.7 

Gold  

...    i  062  .  4 

10295 

10296 

_    | 

—  O.  I 

Copper  

...    1082.6 

10534 

10530 

+  4 

+0.3 

i  206  .  4 

I2OOO 

U988 

+  12 

+  1.0 

1298.5 

I3IOO 

13091 

+  9 

+0.7 

Diopside.  .  .  . 

...     1391-2 

14228 

14215 

+  '3 

+  1.1 

Nickel  

-••     '452-3 

'4977 

14963 

+  '4 

+  1.2 

Cobalt  

...     1489.8 

'5439 

15424 

+  '5 

+  1.2 

Palladium  .  .  . 

...     1549-2 

16143 

16157 

-'4 

-1.2 

Platinum  .... 

.„„„.    | 

18616 

18681 

-* 

-5-4 

MELTING-POINT   OF   PLATINUM.  113 

18.     EXTRAPOLATION  UPWARD.    MELTING-POINT  OF  PLATINUM. 

Now  that  the  gas  thermometer  has  given  us  measured  temperatures  up 
to  1550°,  the  extrapolation  out  to  the  platinum  melting-point,  which  in 
recent  years  has  been  variously  estimated  at  from  1710°  (Harker)  to  1855° 
(Barus),  should  at  least  become  more  certain  than  heretofore.  To  extra- 
polate 200°  beyond  the  upper  limit  of  a  curve  which  has  been  measured 
from  300°  to  1550°  is  a  task  of  quite  different  character  from  the  one 
hitherto  undertaken  of  extrapolating  650°  beyond  a  measured  curve  which 
stopped  at  1 100°. 

Two  methods  are  available  and  convenient  for  making  this  extrapolation . 

1 i )  To  continue  observations  of  the  electromotive  force  of  the  thermo- 
element until  the  platinum  wire  melts. 

(2)  To  employ  one  of  the  radiation  pyrometers  to  measure  the  temper- 
ature interval  from  palladium  to  platinum. 

The  first  method  is  the  most  sensitive  one  available  in  this  temperature 
region  and  inasmuch  as  we  had  been  using  it  continuously  and  successfully 
for  purposes  of  interpolation,  the  opportunity  seemed  favorable  for  adding 
as  good  an  estimate  of  this  classical  temperature  point  as  the  rather.unusual 
facilities  at  our  disposal  permitted.  Platinum,  like  gold,  offers  an  ideal  fixed 
temperature  during  melting.  The  melting-point  is  sharp,  it  can  be  deter- 
mined in  air  without  fear  of  oxidation  or  contamination  either  from  the 
atmosphere  or  from  contact  with  lime,  magnesia,  or  stable  silicates.  More- 
over, the  purity  of  platinum  is  so  readily  checked  by  its  physical  proper- 
ties (electromotive  force  in  thermo-elements,  temperature  coefficient  of 
conductivity)  that  there  is  hardly  a  possibility  of  uncertainty  from  this 
cause.  The  present  determinations  were  made  by  melting  the  platinum 
wire  of  Heraeus  thermo-elements  of  which  the  recorded  electromotive  force 
will  afford  a  sufficient  guarantee  of  its  purity.  The  impurities  contained 
in  these  wires  are  hardly  determinable  quantitatively  by  chemical  means. 

Each  thermo-element  was  heated  gradually  up  to  the  melting-point  of 
the  platinum  wire,  within  a  glazed  Marquardt  porcelain  tube,  in  the  region 
of  maximum  temperature  of  a  resistance  furnace  of  the  carbon  tube  type.1 
Carbon  monoxide  around  the  outside  of  the  porcelain  tube  protected  the 
furnace  from  oxidation,  and  a  current  of  dry  air  in  the  inside  prevented 
contamination  of  the  thermo-element.  Both  wires  of  the  element  were 
inclosed  in  Marquardt  capillaries,  leaving  only  about  2  mm.  of  the  platinum 
exposed  next  to  the  junction.  It  was  always  this  portion  that  melted,  the 
point  being  marked  by  a  halt  of  about  one  minute  in  the  gradual  rise  in 
temperature  of  the  element,  preceding  the  formation  of  a  globule  and  the 
interruption  of  the  circuit.  New  Marquardt  tubes  were  used  for  each 
determination.  Several  elements  were  examined  for  contamination  after 
the  measurement,2  and  no  appreciable  amount  was  found.  A  vertical 
section  of  the  furnace  is  shown  in  Fig.  14. 

In  Table  XVI  are  given  the  experimental  results.  The  values  are  in 
microvolts,  corrected  as  before.3  The  silver  point,  being  about  half  way 

1S.  A.  Tucker.  Trans.  Atner.  Electrochem.  Soc.,  II,  303-306,  1907. 
"Method  described  on  p.  64. 
'Page  63. 


HIGH   TEMPERATURE   GAS   THERMOMETRY. 


between  o°  and  platinum,  is  included  in  order  to  indicate  the  general  course 
of  the  interpolation  curve. 

With  these  are  included  the  results  obtained  by  other  observers  with 
similar  10  per  cent  rhodium  alloys.  Harker1  measured  the  E.  M.  F.  at  the 
melting-point  of  the  platinum  wire  in  a  resistance  furnace  of  refractory 
oxides.  Waidner  and  Burgess2  made  similar  measurements  in  connection 
with  their  optical  determination  of  the  melting-point  of  platinum.  Their 
figures,  being  in  terms  of  the  United  States  legal  volt  (Clark  at  15°=  1.434) 
^  x  have  been  corrected  to  the  true 

volt(Clarkati5°=i.4328).  Other 
investigators  (Holborn  and  Hen- 
ning,3  Nernst  and  von  Warten- 
berg,4  Holborn  and  Valentiner5) 
have  measured  the  melting-point 
of  platinum  in  various  ways,  but 
without  recording  the  thermo- 
electric data. 

In  general,  the  curves  for  all 
the  different  lo.per  cent  elements, 
both  our  own  and  those  of  other 
observers,  are  similar  in  form,  and 
the  divergence  of  each  from  the 
mean  increases  with  increasing 
temperature.  In  this  connection 
it  should  be  remarked  that  the 
E.  M.  F.  at  the  palladium  point 
(16140  microvolts)  obtained  by 
Holborn  and  Valentiner  with  an 
element  whose  gold  point  was 
1 02 95,  agrees  almost  exactly  with 
a  similar  element  from  our  series, 
with  a  gold  point  of  10295  and 
palladium  point  of  16143.  The 
disagreement  between  various  ob- 
servers as  to  the  melting-points  of 
these  metals  is,  then,  not  so  much 
a  matter  of  purity  of  metals  or 
accuracy  of  thermo-electric  meas- 


;eARBORUNDUM, 


V 

I 

:S'v   ™ 
"•V 

_„ 

:VV< 

•••••'.  C/ 

!* 

RBORUNDUM  / 

~ 

n 

:[| 

L 

*=^ 

THERMOELEMENT 


FIG.  14.  Carbon-tube  furnace  for  melting- 
point  of  platinum.     Scale,  i  :  4. 


urements,  as  it  is  of  the  evaluation  of  these  in  terms  of  the  nitrogen  ther- 
mometer. 

We  have  extrapolated  the  curves  of  our  own  elements,  on  which  we  have 
complete  data,  using  for  this  purpose  the  portion  of  the  curve  from  1 100° 
to  1 5 50° .  A  parabola  passed  through  the  melting-points  of  copper  ( 1 08 2 . 6°) , 
diopside  (1391.2°),  and  palladium  (1549.2°),  gives,  in  the  case  of  the  various 
10  per  cent  elements,  values  for  platinum  from  1748°  to  1753°;  the  i  per 


p.  200,   1907. 


'J.  A.  Harker,  Proc.  Roy.  Soc.,  76,  A,  235-250,  1905. 

-C.  W.  Waidner  and  G.  K.  Burgess,  Bull.  Bur.  Standards,  vol  3, 

nL.  Holborn  and  F.  Henning,  Sitzb.  Berl.  Akad.,  1905,  311-317. 

4W.  Nernst  and  H.  von  Wartenberg,  Ber.  Deut.  Phys.  Ges.,  4,  48-58,  1906. 

BL   Holborn  and  S.  Valentiner,  Ann.  Phys.  (4),  22,  1-48,  1907. 


MELTING-POINT   OF   PLATINUM.  115 

cent  alloy  gives  I75o°-i755°  (low  sensitiveness);    the  5  per  cent,  1752°; 
and  the  15  per  cent,  1755°.    The  mean  is  1752°. 

TABLE  XVI. — THERMAL  E.  M.  P.  AT  MELTING-POINT  OF  PLATINUM. 


Date. 

No.  of 
element. 

Rhin 
alloy  wire. 

Silver. 

Platinum. 

Day  and  Bosnian: 

p.  Ct. 

mv. 

mv. 

1910,  19  February  

.    F 

10 

9103 

18619 

24  February  

.    F 

IO 

9103 

18613 

25  February  

.    Y 

IO 

9'39 

18695 

25  February  

.    Z 

IO 

9018 

18487 

8  March  

.    J 

IO 

9106 

18603 

i  March  
ij  April  

•   IJ 
•    KJ 

I 

5 

1960 
6495 

3560 
12444 

2  March  

•    I15J 

I  j 

10375 

22303 

1  March  

•    I,J 

'5 

10375 

22310 

Marker  1905 

N  P  L  3 

IO 

(0084) 

18580 

Harker  1905  

.    T.  .5 

IO 

vyw/i/ 
(9100) 

,8693 

Waidner  and  Burgess,  1907  .  . 

.    P, 

10 

(9024) 

18369 

Waidner  and  Burgess,  1907..  . 

.    P3 

IO 

(9040) 

18556 

Waidner  and  Burgess,  1907..  . 

.  s, 

IO 

(8990 

18250     , 

We  did  not  personally  undertake  any  measurements  with  radiation  pyro- 
meters, nor  has  any  one  yet  had  an  opportunity  to  make  use  of  our  gas- 
thermometer  temperatures  for  this  purpose,  but  optical  determinations  of 
the  temperature  interval  between  palladium  and  platinum  may  be  taken 
from  the  older  observations  without  serious  error  so  long  as  the  absolute 
temperature  values  are  not  required.  Thus,  for  example,  we  find: 

TEMPERATURE  INTERVAL  PALLADIUM  TO  PLATINUM. 

Nernst  and  von  Wartenberg  (Berlin) 204° 

Holborn  and  Valentiner  (at  the  Reichsanstalt) 207° 

Waidner  and  Burgess  (at  the  Bureau  of  Standards) 207° 

Mean 206° 

The  mean  value  of  this  temperature  interval  from  three  good  determina- 
tions is  therefore  206°.  Having  fixed  the  palladium  point  on  the  nitrogen 
thermometer  at  1549°,  if  we  simply  add  206°  to  this  number  we  obtain  a 
second  extrapolated  value  of  the  platinum  point  at  1755°,  in  excellent  agree- 
ment with  the  first. 

The  comparatively  short  interval  over  which  extrapolation  is  now  re- 
quired (i550°-i75o°)  and  the  fact  that  two  wholly  independent  methods 
yield  temperatures  for  melting  platinum  which  differ  but  3°,  gives  to  this 
extrapolation  an  appearance  of  trustworthiness  which  the  earlier  estimates 
have  not  possessed.  The  melting  temperature  of  pure  platinum  may 
therefore  be  considered  fairly  secure  at  1752°  with  an  absolute  error  of 
perhaps  ±5°. 


Il6  HIGH    TEMPERATURE    GAS   THERMOMETRY. 


19.  THE  THERMO-ELEMENT  CURVE  FROM  0°  TO  1755°. 

Below  300°  the  sensitiveness  of  the  platinum-platinrhodium  element  is 
very  low  compared  with  the  platinum  resistance  thermometer,  the  copper- 
constantan  thermo-element,  or  the  mercury  thermometer.  Nevertheless, 
it  is  often  convenient  to  use  an  available  element  for  measurements  in  the 
lower  range;  hence  we  have  in  addition  determined  the  course  of  the  thermo- 
element curve  from  o°  to  300°. 

The  melting  and  boiling  points  of  pure  substances  determined  on  the  gas 
thermometer  form  the  basis  of  this  calibration  as  before.  The  o°  and  100° 
points  are  familiar.  In  the  neighborhood  of  200°  and  300°  the  boiling- 
points  of  pure  naphthalene  and  benzophenon  were  used.  The  only  gas- 
thermometer  determinations  of  these  two  points  since  the  early  and  some- 
what less  accurate  measurements  by  Crafts,1  namely,  those  of  Jaquerod  and 
Wassmer,2  differ  from  the  values  interpolated  by  Callendar  and  Griffiths3 
with  the  resistance  thermometer,  by  0.26°  at  218°  and  0.4°  at  305°.  The 
values  of  Jaquerod  and  Wassmer,  which  we  have  used,  are  217.68°  for 
naphthalene  and  305.44°  for  benzophenon,  at  760  mm.  pressure.4 

In  our  work  with  the  gas  thermometer,  one  measurement  was  made  at  the 
melting-point  of  cadmium,  to  give  an  indication  of  the  course  of  the  thermo- 
element curve  in  this  lower  region.  Being  only  a  single  measurement,  this 
has  not  as  much  weight  as  the  higher  temperatures,  which  were  measured 
under  varied  conditions.  The  value  obtained  was  320.0°. 

The  difference  between  benzophenon  and  cadmium,  determined  with 
three  platinum-platinrhodium  thermo-elements,  was  found  to  be  14.8°.  The 
benzophenon  used  was  Merck's  preparation,  which  boils  0.2°  higher  than 
the  purest  made  by  Kahlbaum.  The  difference  between  Kahlbaum's 
benzophenon  and  cadmium  is  then  15.0°,  which  is  exactly  the  difference 
found  by  Waidner  and  Burgess  at  the  Bureau  of  Standards,  using  a  resist- 
ance thermometer.5  On  the  basis  of  the  benzophenon  value  adopted  above, 
this  difference  makes  the  cadmium  point  320.4°.  We  have  arbitrarily  con- 
nected the  two  portions  of  the  temperature  scale  at  this  point  by  taking  the 
mean,  320.2°,  for  cadmium.  Since  in  this  region  temperatures  can  not  be 
coveniently  obtained  closer  than  0.2°  with  the  platinum-platinrhodium 
element,  the  values  are  abundantly  accurate  for  the  present  purpose. 

It  should  not  be  overlooked  that  the  value  which  we  have  obtained  for 
zinc  indicates  a  lower  value  for  the  boiling-point  of  sulphur  than  the  figure 
444.5°  now  in  general  use.  The  four  independent  gas-thermometer  deter- 
minations that  have  been  made  of  the  sulphur  point,  although  agreeing 
unusually  well,  are  not  free  from  the  possibility  of  errors  of  several  tenths 
of  a  degree,  and  this  fact,  taken  together  with  the  variability  in  the  point 
itself  with  different  experimental  conditions,  makes  it  probable  that  the 
absolute  value  given  for  the  sulphur  point  is. still  somewhat  uncertain. 

'Bull.  Soc.  Chim.,  39,  277-289,  1883. 

5Jour.  Chim.  Phys.,  2,  52-78,  1904. 

"Phil.  Trans.  Roy.  Soc.,  182,  A,  43-72,  119-157,  1891. 

4The  results  of  Jaquerod  and  Wassmer  have  also  been  used  as  the  standard  since  1904  by  the  Research 
Laboratory  of  Physical  Chemistry  at  the  Massachusetts  Institute  of  Technology,  in  their  work  on  electrical 
conductivity  at  high  temperatures. 

"Bull.  Bur.  Stds.,  7,  1-9,  1910. 


THE  THERMOELEMENT  CURVE  FROM  o    TO  1755  . 


117 


THERMO-ELEMENTS  IN  EVERY-DAY  PRACTICE. 

The  summarized  temperature  scale  adopted  for  present  use  in  this  labora- 
tory for  the  calibration  of  thermo-elements  is  therefore  as  follows : 


Ice,  m.  p 

Water,  b.  p 

Napthalene,  b.  p.  .  . 
Benzophenon,  b.  p. . 

Cadmium,  m.  p 

Zinc,  m.  p 

Antimony, m.p(inCO)  629.2 
Silver,  m.  p.  (in  CO) .     960 .  o 


100.0+0.037  (p  — 760) 
217.7+0.057  (p  — 760) 
305.4+0.063  (p— 760) 
320.2 
418.2 


Gold,  in.  p 1062  .4° 

Copper,  m.  p.  (in  CO) 1082 . 6 

Li2SiO3 1 20 1  . 

Diopside,  m.  p 1391 .2 

Nickel,  m.  p.  (in  No) 1452.3 

Cobalt,  m.  p.  (in  N2) 1489 . 8 

Palladium,  m.  p 1549-2 

Platinum,  m.  p 1755  . 


600"   700J   800°   900°  1000  "'  1100  '1 

TEMPERATURE 


1300  ;  1  400    15003  IWKT  1;00°1755 


FIG. 


15.  Deviation  of  typical  thermo-elements  from  standard  curve. 


For  interpolation  in  every-day  practice  over  this  long  range  of  temper- 
atures we  may  use  either  an  empirical  equation  or  series  of  equations,  or  we 
may  plot  the  temperatures  and  microvolts  and  draw  a  smooth  curve  through 
the  points.  The  results  of  one  method  have  no  better  claim  to  accuracy 
than  the  results  of  the  other,  for  an  empirical  equation  is  essentially  noth- 
ing but  an  imaginary  curved  ruler.  A  plotted  curve  on  a  scale  large  enough 
to  get  the  requisite  accuracy  of  reading  would,  however,  take  a  sheet  at 
least  30  feet  square.  But  if  instead  of  plotting  microvolts  directly  against 
degrees,  we  plot  the  deviation  from  the  straight  line,  e=  lot,  the  sheet  re- 
quired is  reduced  to  about  3  feet  square.  If  further,  we  plot  the  deviations 
of  each  element  from  an  arbitrary  standard  curve,  instead  of  the  deviations 
from  a  straight  line,  the  usual  50  cm.  x  40  cm.  sheet  is  ample. 

The  figures  of  Table  XVII  represent  such  a  curve,  which  lies  very  close 
to  the  actual  curve  for  the  standard  thei  mo-element  E  used  in  the  work  on 
the  nitrogen  thermometer.  The  deviations  of  various  other  elements,  in 
use  in  the  laboratory,  from  this  standard  are  plotted  in  Fig.  15.  These 
curves  are  obtained  by  plotting  the  differences  between  the  reading  of  the 
element  and  that  of  the  assumed  standard  at  each  calibration  point. 

An  example  will  serve  to  make  clear  the  method  of  converting  microvolts 
into  degrees  with  this  table  and  curve.  It  is  desired  to  find  the  tempeiature 
corresponding  to  a  reading  of  8931  microvolts  on  element  Z.  It  is  evident 
from  the  table  that  the  temperature  is  in  the  neighborhood  of  950°.  At 
about  this  temperature,  element  Z  reads  92  microvolts  below  the  assumed 
standard;  adding  92  microvolts  to  8931  gives  9023  microvolts  as  the  corre- 
sponding standard  reading.and  this  by  interpolation  in  the  tablegives  952.3°. 
The  tenths,  of  course,  mean  little  in  absolute  value;  but  temperature 
differences,  in  case  measurements  are  made  with  similar  elements  under 
similar  conditions,  can  be  obtained  if  need  be  to  tenths  of  a  degree. 


n8  HIGH  TEMPERATURE;  GAS  THERMOMETRY. 

TABLE  XVII. — STANDARD  CURVE  OK  ELEMENT  PT:  (90  PT  10  RH)  FROM  o°  TO  1755° 


/ 

e 

Diff. 

t 

e 

Diff. 

I 

e 

Diff 

0 

300 

2315 

580 

5026 

55 

92 

IOI 

10 

55 

305.4  Benzo. 

2)6^ 

590 

5I27 

57 

102 

20 

112 

310 

2407 

600 

5229 

60 

93 

102 

30 

172 

320 

2500 

610 

5331 

62 

93 

103 

40 

234 

320.2  Cd 

2502 

620 

5434 

63 

103 

5<> 

297 

330 

2593 

629.2 

Sb  5529 

65 

93 

60 

362 

340 

2687 

630 

5537 

67 

94 

103 

70 

429 

350 

2781 

640 

5640 

69  ; 

94 

104 

80 

498 

!36o 

2875 

650 

5744 

7« 

94 

104 

90 

569 

370 

2969 

660 

5848 

72 

95 

104 

100 

64I 

;38o 

3064 

670 

5952 

73  ! 

95 

104 

no 

7'4 

390 

3  '59 

680 

6056 

75 

95 

10j 

120 

789 

400 

3254 

690 

6l6l 

77 

96 

105 

130 

866 

410 

3350 

700 

6266 

78 

96 

105 

140 

944 

418.2   Zn 

3429 

710 

6371 

79 

1  06 

150 

1023 

420 

3446 

720 

6477 

80 

96 

1  06 

160 

1  103 

430 

3542 

730 

6583 

1 

81 

97 

1  06 

,70 

1184 

440 

3639 

740 

6689 

82 

97 

1  06 

1  80 

1266 

450 

3736 

750 

6795 

83 

97 

107 

190 

1349 

460 

3833 

760 

6902 

84 

98 

107 

200 

'433 

470 

393" 

7/0 

7009 

85 

98 

1  08 

2IO 

1518 

480 

4029 

780 

7117 

98 

108 

217.7 

Napht.  1584 

490 

4127 

790 

7225 

86 

99 

108 

220 

1604 

500 

4226 

800 

7333 

86 

99 

1  08 

230 

1690 

510 

4325 

810 

7441 

87  i 

99 

109 

240 

1777 

i  520 

4424 

820 

7550 

88 

IOO 

109 

250 

1865 

530 

4524 

830 

7659 

89 

IOO 

I  IO 

260 

1954 

i  540 

4624 

840 

7769 

89 

IOO 

I  IO 

270 

2043 

550 

4724 

850 

7879 

9° 

IOO 

I  10 

280 

2133 

560 

4824 

860 

7989 

91 

IOI 

1  1  1 

290 

2224 

570 

4925 

870 

8100 

9' 

IOI 

III 

THE   THERMO-ELEMENT  CURVE   PROM  O°   TO    1755' 


IIQ 


TABLE  XVII. — STANDARD  CURVE  OF  ELEMENT  PT:  (90  PT  ioRn)  FROM  o°  TO  1755°— 

Concluded. 


1 

e 

Diff. 

i 

e   Di 

if. 

1 

e  Di 

a. 

88o 
890 
900 
910 
920 
930 
940 
950 
960 
960  .0  Ag 
970 
980 
990 

8211 
8322 
8434 
8546 
8658 
8771 
8884 
8997 
91  I  1 
gin 
9225 
9339 
9454 

1  12 
I  12 
I  12 

"3 

"3  ! 

"4  ; 

"4 

"4 

"5 

1  170 
1180 
1  190 

12OO 
12IO 
1220 
123O 
I24O 
1250 
1260 
127O 
1280 
1290 

11572 
I  1692 
Il8l2 
11932 
I2O52 
12172 
12292 
12412 
12532 
12652 
12772 
12892 
13012 

120 
12O 
120 
120 
1  2O 
!2O 
120 
1  2O 
I2O 
120 
120 
I2O 

1470 
1480 

1489.  8  Co 
1490 
1500 
1510 
1520 
1530 
1540 

'550 
1560 
'570 

15183 
15304 
15423 
15425 
15546 
15666 
15787 
15908 
16029 
16140 
16150 
16270 
16391 

121 
121 

121 
12O 
121 
121 
121 
121 

120 
121 

"5 

1  2O 

121 

IOOO 

9569 

I3OO 

13132 

1580 

16512 

1  16 

120 

120 

1010 

9685 

13IO 

13252 

1590 

16632 

116  j 

1  2O 

121 

1020 

9801 

132O 

'3372 

1600 

16753 

116 

12O 

I2O 

IO3O 

9917 

1330 

13492 

1610 

16873 

1  17 

120 

120 

IO4O 

10034 

1340 

13612 

1620 

16993 

117 

121 

12O 

1050 

10151 

1350 

'3733 

1630 

I7II3 

"7 

121 

120 

IO6O 

10268 

1360 

13854 

1640 

'7233 

118 

121 

I2O 

1062.4  Att 

10296 

'370 

'3975 

1650 

'7353 

1  2O 

I2O 

1070 

10386 

1380 

14095 

1660 

'7473 

118 

121 

120 

1080 

10504 

1390 

14216 

1670 

'7593 

118 

121 

1  2O 

1082.6  Cu 

10535 

1391  .2  Diops. 

14231 

1680 

17713 

120 

1090 

10622 

1400 

'4337 

1690 

'7833 

118 

121 

12O 

I  IOO 

10740 

1410 

14458 

1700 

'7953 

1 

118 

121 

120 

I  I  IO 

10858 

1420 

'4579 

1710 

18073 

1  19 

120 

120 

1120 

10977 

1430 

14699 

1720 

'8193 

"9 

121 

I2O 

1  130 

1  1096 

1440 

14820 

'730 

,83,3 

i  1Q 

121 

1  2O 

1  140 

11215 

iy 

1450 

14941 

'74° 

18433 

1  19 

121 

1  2O 

1  150 

"334 

1452.3  AT«' 

14969 

1750 

'8553 

1  l6o 

"453 

"9 

,460 

15062 

7755-  Pt 

18613 

12O 

"9 

121 

120  HIGH   TEMPERATURE   GAS  THERMOMETRY. 

The  use  of  Table  XVII  and  the  deviation-curve  (Fig.  15)  avoids  the  cal- 
culation and  recalculation  of  thermo-element  curves  and  the  tabulation  of 
their  readings.  If  the  calibration  of  an  element  changes  by  a  few  micro- 
volts, the  deviation- curve  is  merely  raised  or  lowered  by  a  corresponding 
amount.  If  the  value  adopted  for  one  of  the  calibration  points  is  changed, 
the  corresponding  reading  in  microvolts  of  the  assumed  standard  is  also 
changed,  and  all  the  deviation-curves  take  a  slightly  different  course  in  the 
neighborhood  of  that  point.  The  table  and  curves  make  it  possible,  further- 
more, to  estimate  temperatures  (with  an  accuracy  of  perhaps  5°)  with  a 
new  thermo-element,  by  simply  calibrating  it  at,  say,  two  points  such  as 
silver  and  diopside,  and  thus  locating  it  among  the  family  of  deviation-curves. 

20.  RELATION  OF  THERMAL  E.  M.  F.  TO  COMPOSITION. 

In  the  course  of  the  work  on  the  nitrogen  thermometer,  the  standard  10 
per  cent  elements  were  compared  with  elements  whose  alloy  wires  contained 
i  per  cent  and  15  per  cent  rhodium.  The  K.  M.  F.  of  the  20  per  cent  alloy, 
of  which  the  bulb  was  made,  was  determined  by  two  methods,1  in  order  to 
evaluate  the  different  readings  on  the  nitrogen- thermometer  bulb.  To  make 
the  series  more  complete,  a  5  per  cent  alloy  was  obtained  from  Heraeus  and 
its  readings  against  pure  platinum  were  compared  with  the  standards. 

A  similar  series  of  comparisons  was  made  in  1892  by  Holborn  and  Wien,2 
using  alloys  with  9,  10,  n,  15,  20,  30,  40,  and  100  per  cent  rhodium.  This 
work  was  done,  however,  just  at  the  beginning  of  the  careful  work  of  Mylius 
on  the  separation  of  the  platinum  metals,  and  the  alloys  then  available  were 
not  pure.  In  the  lower  percentage  alloys,  different  elements  of  the  same 
nominal  composition  gave  E.  M.  F.'s  differing  by  10  per  cent  or  more,  and 
varying  differently  with  temperature.  In  the  higher  percentages,  the 
K.  M.  F.  varies  little  with  the  composition,  and  the  results  have  therefore 
some  value  in  indicating  the  course  of  the  curve  of  E.  M.  F.  and  composi- 
tion. The  data  have  been  corrected  to  our  temperature  scale,  and  also  for 
the  difference  in  E.  M.  F.  standards. 

Holborn  and  Day3  in  1899  obtained  the  E.  M.  F.  of  pure  platinum  against 
two  samples  of  pure  rhodium  up  to  1300°.  The  data  have  been  corrected 
to  correspond  to  our  temperature  scale. 

Waidner  and  Burgess4  measured  the  E.  M.  F.  of  the  10  per  cent  against 
the  20  per  cent  alloy,  at  various  points  up  to  1755°,  using  two  samples. 
The  addition  of  this  value  to  the  E.  M.  F.  of  the  10  per  cent  alloy  against 
pure  platinum  gives  an  independent  check  on  our  direct  measurements  with 
the  20  per  cent  alloy.  As  shown  by  Fig.  16,  the  agreement  is  very  good. 

The  summarized  data  are  given  in  Table  XVIII.  For  the  10  per  cent 
alloy  three  values  are  given :  first,  the  lowest-reading  of  the  twelve  elements 
used  with  the  nitrogen  thermometer;  second,  the  highest-reading;  and 
third,  the  standard  element  E. 

The  frequent  comparisons  of  the  platinum' and  platinrhodium  wires  of  the 
standard  10  per  cent  elements  duiing  the  work  on  the  nitrogen  thermometer, 
show  that  the  differences  among  them  are  due  partly  to  the  platinum  wire 

'Page  67. 

aL.  Holborn  and  W.  Wien,  Uber  die  Messung  hoher  Temperaturen,  Ann.  Phys.,  47,  107-134,  1892. 
3Thermo-electricity  in  certain  metals,  Amer.  Jour.  Sci.  (4),  8,  303-308,  1899;  Ann.  d.  Phys.  2,  p.  522,  1900. 
Sitzb.  Berl.  Akad..  1899,  691-695. 
*Bull.  Bur.  Stds,.  3,  p.  200,  1907. 


RELATION    Ol?   THERMAL   E.    M.    F.    TO   COMPOSITION.  121 

and  partly  to  the  alloy.  Element  Z,  for  instance,  reads  lower  than  E  chiefly 
because  the  platinum  wire  of  Z  is  more  impure  than  that  of  E;  the  effect 
of  this  impurity  is  partly  neutralized  by  an  apparently  larger  amount  of 
rhodium  in  the  alloy  wire.  This  appears  from  the  data  in  the  table  below, 
which  show  comparisons  between  several  typical  10  per  cent  elements. 
The  purest  platinum  appears  to  be  that  of  J.  If  the  thermo-electric  effect 
of  rhodium  is  proportional  to  its  percentage  from  o  to  i  per  cent,  then  about 
o .  05  per  cent  rhodium  in  platinum  wire  would  be  sufficient  to  produce  the 
difference  between  Z  and  E.  The  data  are  in  microvolts. 

TABLE  XVIII. — THERMAL  E.  M.  F.  OF  PURE  PLATINUM  AGAINST  PLATINUM-RHODIUM 
ALLOYS,  IN  MILLIVOLTS. 


10  p.  Ct. 

t 

IP 

.  ct. 

5  p.  ct. 

15  p.  ct. 

20  p.  Ct. 

aop.ct.1 

40  p.  ct.1 

100  p.  ct.* 

Low. 

High. 

Stand- 
,     ard. 

100° 

0 

2  I 

0.55 

0.63 

0.64 

0.64 

0.65 

0.65 

2OO 

o. 

42 

1.18 

1.41 

1-43 

'•43 

1.50 

1.51 

300 

0. 

63 

1.85 

2.28 

2.32 

2.32 

2.41 

2-34 

2.4, 

2-57 

400 

o. 

<S4 

2-53 

3  -21 

3.26 

3-2? 

3  45 

3.50 

3.50 

3-64 

3  76 

500 

OS 

3.22 

4-  '7 

4-23 

4.23 

4-55 

4.60 

•  4-74 

4-93 

5.08 

600 

2s 

3.92 

5.16 

5-24 

5  23 

5-7' 

5-83 

6.06 

6.31 

6.55 

7OO 

45 

4.62 

6.  19 

6.28 

6.27 

6.94 

7-18 

7-49 

7.80 

8.,4 

800 

6; 

5-33 

7-25 

7-35 

7-33 

8.23 

8.60 

Q.OI 

9  37 

1    9.87 

900 

8s 

6.05 

8.35 

8.46 

8.43 

9-57 

10.09 

10.67 

1  1  .09 

11.74 

IOOO 

2 

OS 

6.79 

9-47 

9.60 

9  57 

10.96 

I  I  .65 

12.42 

12.94 

13-74 

1100 

2 

2S 

7-53 

10.64 

10.77 

10.74 

12.40 

13-29 

M-33 

'4-99 

15.87 

I2OO 

2 

4:, 

8.29 

11.82 

11.97 

11.93 

13.87 

14.96 

16.39 

17.13 

18.  10 

13OO 

2 

65 

9.06 

13.02 

.3-18 

13.13 

15.38 

16.65 

18.51 

19.51 

20.46 

I4OO 

2 

86 

9.82 

14.22 

'4-39 

'4-34 

16.98 

18.39 

20.67 

21.73 

15OO 

=* 

06 

10.56 

15-43 

.5.61 

'5-55 

18.41 

2O.  15 

1600 

} 

26 

11.31 

16.63 

16.82 

16.75 

19.94 

21  .90 

17OO 

} 

46 

12.05 

17.83 

18.03 

17-95 

21-47 

23.65 

-/ 

I  8    AO 

1  8  "o 

18  61 

'757 

} 

! 

'Holborn  and  Wien,  1892,  loc.  cit. 


a  Holborn  and  Day,  mean  value,  1899,  loc.  cit. 


The  data  of  Table  XVIII  are  plotted  in  Fig.  16,  which  shows  the  relation 
between  temperature  and  thermal  E.  M.  F.  for  various  alloys.  The  30  per 
cent  and  40  per  cent  curves  represent  the  data  of  Holborn  and  Wien.  The 
curve  for  pure  rhodium  represents  the  mean  of  the  two  samples  by  Holborn 
and  Day.  There  is  no  indication  of  a  break  in  any  of  the  curves  over  the 
entire  range  of  temperature. 

In  Fig.  17,  the  data  of  Table  XVIII  are  plotted  to  show  the  relation  of 
the  thermal  E.  M.  F.  at  various  constant  temperatures  to  the  composition 
of  the  alloy  wire,  the  cold  junction  being  in  every  case  at  o°.  At  all  tem- 


E.  M.  F.  of  Pt 

E.  M.  F.  of  Pt-Rh 

Difference 

Element. 

wire  against  Pt 
of  E  at  1500°. 

wire  against  Pt-Rh 
of  E  at  1500°. 

between 
elements. 

Y 

+     12 

+75 

+   63 

Z 

+  •77 

+67 

—  HO 

A 

+  75 

+47 

—    28 

F 

+     7 

+   i 

—     6 

J 

-     9 

+   i 

+   10 

HIGH   TEMPERATURE    GAS   THERMOMETRY. 


peratures  the  E.  M.  F.  increases  very  rapidly  with  the  first  additions  of 
rhodium,  and  at  20  per  cent  the  value  has  already  reached  81  to  93  per  cent 
of  the  E.  M.  F.  of  platinum  against  pure  rhodium. 


z 


&--JJL 


VV 


A 


I   15 


LL 


///A 


100°  200°  3003   400'   500°  600°  700°  800°  900"  1000"  1100°  1200°  1300°  1400°  1500°  1(500 1700  1755 
TEMPERATURE. 

FIG.  16.  Relation  of  temperature  to  thermal  electro-motive  force  of  platinum  against  platinum- 
rhodium  alloys. 

The  thermo-electric  power,  or  rate  of  change  of  E.  M.  F.  with  temperature 
de  . 
~T  is  plotted  in  Fig.  18,  against  the  atomic  concentration  of  the  alloy.    The 

values  are  in  microvolts,  against  pure  platinum.    The  curves  for  all  tern- 


RELATION   OF   THERMAL   E.    M.    F.    TO    COMPOSITION. 


123 


peratures  are  similar  in  form  and  approach  the  curve  for  1755°  as  an 
envelope. 

In  a  recent  study  of  the  thermo-electric  properties  at  low  temperatures 
of  the  alloys  of  tellurium  with  antimony,  tin,  and  bismuth,  and  of  antimony 
with  silver,  Haken '  comes  to  the  conclusion  that  a  thermo-electric  curve  of 
the  form  of  those  in  Fig.  18  accompanies  the  formation  of  a  solid  solution 
between  the  end  components,  while  compounds  are  marked  by  sharp  max- 
ima or  minima.  The  thermo-electric  curves  of  the  systems  copper-cobalt, 
by  Reichardt;2  copper-nickel,  by  Feussner  and  Ljndeck;3  and  silver-zinc,  by 
Puschin  and  Maximenko,4  show  a  similar  relationship  between  the  form  of 
the  curve  and  the  constitution  of  the  alloy.  More  recently,  E.  Rudolfi5 
has  published  a  study  of  these  relationships  for  eight  pairs  of  metals. 


±2 


Holborn  «nd  Dau  I  a 
Waidner  anJ  Bu'roess  x 
Ho/born  and  Men  A 


10       10       30       40        50       60       70       80 

PIG.  17.  Relation  of  thermal  electromotive  force,  e,  to  com 
sition  of  platinum-rhodium  alloys. 


num,  to  atomic  composition. 


The  alloys  of  platinum  and  rhodium  have  not  been  studied  microscopic- 
ally or  thermally,  but  measurements  in  our  carbon-tube  furnace  showed  that 
the  melting-points  of  the  i  per  cent  and  5  per  cent  alloys  are  higher  than 
1755°.  The  melting-point  of  the  10  per  cent  alloy  is  given  by  von 
Wartenberg6  as  1830°,  and  of  pure  rhodium  as  1940°.  It  is  very  probable, 
therefore,  that  platinum  and  rhodium  form  solid  solutions  at  least  as  far  as 
55  atomic  per  cent  rhodium,  with  no  compounds,  over  the  range  of  tem- 
perature covered  by  our  data. 

'Verh.  deutsch.  phys.  Ges.  12,  229-239,  1910.  Ann.  d.  phys.,  32,  291-336,  1910. 

2Ann.  d.  Phys.,  6,  832-55,  1901. 

3Wiss.  Abh.  Phys. -Tech.  Reichsanstalt,  2,  p.  515,  1895. 

'Jour  Russ.  Phys.  Chem.  Ges.,  41,  500-524,  1909. 

"Zeitschr.  anorg.  Chem.,  67,  65-96,  1910. 

"Verb.  Deutsch.  Phys.  Ges.,  12,  121-127,  1910. 


124  HIGH  TEMPERATURE  GAS  THERMOMETRY. 


21.  SUMMARY. 

It  is  now  some  thing  over  five  years  since  the  Geophysical  Laboratory  took 
up  the  task  of  redetermining  the  absolute  temperature  scale  from  400°  to  1 1 00° 
with  the  nitrogen  thermometer,  and  of  extending  it,  if  it  should  prove  prac- 
ticable to  do  so,  to  1600°  C.,  covering  the  region  in  which  are  found  most 
of  the  mineral  relations  which  it  is  the  chief  purpose  of  the  laboratory  to 
study.  Two  preliminary  publications  have  been  made  during  the  investi- 
gation. One,  a  brief  summary  of  preliminary  work  up  to  1 100°,  was  given 
before  the  National  Academy  of  Sciences  and  the  American  Physical  So- 
ciety in  April,  1907;'  the  second  covered  the  same  ground  at  considerable 
length  in  1908. 2  A  final  paper3  extending  the  observations  to  1550°, 
and  a  supplementary  paper4  on  the  use  of  thermo-elements  throughout  the 
entire  range  from  zero  to  melting  platinum  appeared  in  1910  and  completed 
the  work  contemplated  under  the  original  plan. 

The  gas-thermometer  problem  at  the  present  stage  of  its  development 
has  become  primarily  a  problem  for  experimental  study  with  two  definite 
purposes,  one  to  increase  the  accuracy  of  the  measurements,  the  other  to 
increase  their  range.  The  application  of  the  gas  laws  is  no  longer  subject  to 
serious  question.  The  progress  of  recent  years  has  given  us  electric  heating 
in  place  of  gas  and  the  consequent  possibility  of  controlling  the  temperature 
with  great  certainty  and  exactness.  It  has  also  given  us  the  metal  bulb 
with  a  definite  and  measurable  expansion  coefficient  and  capable  of  holding 
the  expanding  gas  without  loss.  It  has  discovered  a  gas  which  does  not 
diffuse  through  the  bulb  or  react  with  it  chemically,  which  does  not  dis- 
sociate within  the  limits  of  practicable  measurement,  and  of  which  the  ex- 
pansion can  be  expressed  with  reasonable  certainty  in  terms  of  the  Kelvin 
thermodynamic  scale  whenever  it  may  prove  necessary  or  desirable  to  do  so. 
It  has  also  discovered  the  source  of  the  errors  in  the  thermo-elements  and  a 
way  to  avoid  them. 

In  1904  Professor  Holborn  of  the  Reichsanstalt  increased  the  range  of 
this  scale  as  far  as  1600°  C.,  the  probable  error  of  the  new  portion  being  10°. 
Simultaneously  with  this  effort,  work  was  begun  at  the  Geophysical  Labora- 
tory in  Washington  with  a  view  to  increasing  the  accuracy  of  the  scale, 
first  over  the  existing  range  (to  1 150°),  and  later,  as  much  beyond  this  point 
as  it  should  prove  practicable  to  go. 

No  attempt  will  be  made  to  offer  an  inclusive  summary  of  the  whole 
investigation.  It  is  a  record  of  experimental  measurements  covering  an 
unusually  wide  range  of  details  which  do  not  admit  of  brief  classification. 
The  errors  which  have  heretofore  been  present  in  measurements  with  the 
nitrogen  thermometer  have  been  reduced  by  the  present  investigation  to 
about  one-fourth  their  former  magnitude  and  the  certainty  of  their  evalu- 
ation is  at  least  proportionately  increased. 

The  particular  points  to  which  we  have  given  most  attention  are  the  fol- 
lowing : 

(i)  To  provide  a  uniform  temperature  about  the  bulb  by  a  suitable 
arrangement  of  electric-heating  coils  and  diaframs. 

"Abstract,  Phys.  Rev..  24,  531,  1907.  3Atner.  Jour.  Sci.  (4).  29,  93-161,  1910. 

-Amer.  Jour.  Sci.  (4),  26,  405-463,  1908.  4Amer.  Jour.  Sci.  (4),  30,  1-15,  1910. 


SUMMARY.  125 

(2)  To  inclose  the  furnace  in  a  gas-tight  bomb  in  which  the  pressure  out- 
side the  bulb  can  be  maintained  equal  to  that  within  for  all  temperatures. 
This  offers  three  distinct  advantages :    (a)  it  provides  against  the  deforma- 
tion of  the  bulb  through  differences  of  pressure  within  and  without  in  the 
region  of  highest  temperatures,  where  the  bulb  material  becomes  softer; 
(b)  by  using  the  same  gas  within  and  without,  there  is  no  tendency  for  it 
to  diffuse  through  the  bulb  wall;  (c)  it  enables  the  initial  pressure  to  be 
varied  within  considerable  limits,  thereby  increasing  both  the  scope  and 
sensitiveness  of  the  manometer.    The  sensitiveness  in  our  instrument  with 
this  arrangement  was  about  three  times  that  of  the  Reichsanstalt. 

(3)  The  expansion  of  the  bulb  material  was  determined  with  great  care 
and  is  probably  accurate  within  0.5  per  cent. 

(4)  The  unheated  space  between  the  bulb  and  manometer  has  been  re- 
duced until  the  total  correction  in  this  hitherto  uncertain  region  amounts 
to  less  than  4°  at  1 100°.    An  error  of  5  per  cent  in  the  determination  of  its 
volume  or  temperature  distribution  is,  therefore,  practically  negligible. 

It  is  probable  that  these  changes  serve  to  reduce  the  uncertainty  hitherto 
prevailing  in  the  correction  factors  which  require  to  be  applied  to  the  gas 
thermometer  in  the  region  of  1100°  to  less  than  one-tenth  of  its  former 
magnitude.  Furthermore,  these  improvements  are-  equally  applicable 
throughout  the  region  above  1100°  as  far  as  the  present  measurements 
have  extended  (to  1550°). 

The  chief  source  of  present  uncertainty  is  the  temperature  distribution 
over  the  surface  of  the  bulb  in  an  air  bath.  It  would  be  possible  to  eliminate 
this  error  in  the  lower  portion  of  the  scale  by  substituting  a  liquid  bath  which 
could  be  stirred.  In  fact,  this  was  done  for  temperatures  below  500°  in  the 
earlier  work  of  Holborn  and  Day,  but  has  not  so  far  been  tried  in  the  present 
investigation  because  of  the  relatively  secondary  importance  of  the  lower 
temperatures  to  the  ultimate  purpose  of  the  investigation  (the  study  of 
silicates).  For  the  higher  temperatures  no  satisfactory  liquid  bath  has 
been  found. 

The  treatment  of  experimental  errors  in  a  complicated  problem  of  this 
kind  is  obviously  a  matter  into  which  the  personal  equation  enters  largely. 
The  only  sure  method  is  to  make  their  total  magnitude  disappearingly  small, 
and  to  this  end  our  efforts  have  been  directed  wherever  possible.  Errors 
due  to  failure  of  the  pressure  to  return  to  its  initial  zero  after  heating  to  a 
high  temperature,  which  are  due  to  permanent  changes  in  the  volume  of  the 
bulb,  or  to  absorption  or  loss  of  gas,  are  entirely  negligible  in  the  present 
instrument.  This  source  of  error  is  of  a  particularly  intangible  kind  and 
has  clung  to  all  the  earlier  work  in  gas  thermometry  like  a  haunting  evil 
spirit. 

Another  classical  source  of  uncertainty  to  which  attention  has  already 
been  called,  lies  in  the  unheated  connecting  link  between  the  hot  bulb  and 
cold  manometer  in  which  the  volume  and  temperature  conditions  have  been 
exceedingly  difficult  to  establish.  It  is  also  reduced  to  practically  insig- 
nificant magnitude  in  the  present  instrument  and  can  be  still  further  dimin- 
ished if  necessary. 

Still  a  third  difficulty  which  was  discovered  in  this  laboratory1  soon  after 

'W.  P.  White,  "The  Constancy  of  Thermo-elements,"  Phys.  Rev.,  23,  449-474,  1906. 


126  HIGH  TEMPERATURE;  GAS  THERMOMETRY. 

this  investigation  began  and  which  at  once  assumed  serious  proportions, 
arises  from  the  gradual  contamination  of  the  auxiliary  thermo-elements  by 
iridium  vapor  from  the  heating  coil,  the  bulb,  or  any  platinum  crucibles  or 
diaframs  containing  this  metal  which  are  exposed  by  accident  or  design  in 
the  furnace  at  extreme  temperatures.  This  absorption  of  iridium  vapor 
(in  a  minor  degree  of  rhodium  vapor  also)  which  has  been  an  important 
though  unrecognized  factor  in  all  previous  work  in  which  thermo-elements 
were  used  (beginning  with  Barus),  has  the  effect  of  making  the  thermo- 
elements read  lower  as  contamination  increases.  These  effects  also  have 
been  reduced  to  negligible  magnitude  in  all  the  observations  of  the  present 
series  subsequent  to  the  preliminary  set  (Table  II),  which  has  not  been 
used  in  calculating  the  results. 

The  remaining  correction  factors  are  numerous,  but  with  the  exception  of 
the  expansion  of  the  bulb,  which  has  been  very  carefully  determined,  are 
not  only  small  but  are  unlikely  to  become  cumulative  in  a  particular  case. 
The  temperature  correction  for  the  mercury  columns  of  the  manometer, 
for  example,  which  is  of  a  magnitude  to  catch  the  eye  in  the  table  on  p.  69, 
and  to  which  our  attention  has  been  explicitly  called,  is  in  fact  an  error  of 
very  harmless  character  in  practice  in  the  magnitude  in  which  it  here  enters. 
It  appears  as  an  error  of  the  same  order  of  magnitude  and  same  sign  in  two 
quantities,  the  difference  of  which  is  used  in  the  computation  of  temperature. 
It  varies  but  little  in  the  observations  of  a  single  day  and  does  not  increase 
with  the  temperature.  There  is  therefore  little  of  real  uncertainty  to  be 
apprehended  from  this  source.  The  same  is  true  of  many  of  the  other  minor 
corrections  to  which  the  final  values  are  subject. 

In  general  it  may  be  said,  for  the  information  of  those  who  have  not  a 
first-hand  acquaintance  with  the  gas  thermometer,  that  the  danger  lies  in 
relations  which  become  more  uncertain  as  the  temperature  advances,  such 
for  example  as  the  distribution  of  temperature  about  the  bulb,  which  is 
continually  changing  in  an  air  bath  at  extreme  temperatures,  and  in*the 
expansion  of  the  bulb  material  in  the  same  region.  Changes  in  the  tem- 
perature of  the  bulb  wall  can  be  observed  by  grouping  a  number  of  thermo- 
elements about  it  (eight  were  used  for  this  purpose  in  our  experiments)  and 
corrections  can  be  applied  for  the  differences  observed.  In  the  experiments 
here  recorded  the  temperature  control  was  very  perfect,  even  at  the  highest 
temperatures,  and  differences  of  temperature  on  the  surface  of  the  bulb 
amounting  to  a  whole  degree  were  rarely  observed. 

It  is,  however,  a  little  difficult  to  make  positive  assertions  about  the  tem- 
perature of  the  gas  in  a  containing  vessel  when  the  temperature  on  its  sur- 
face shows  differences  of  10  degrees  or  more,  as  has  usually  been  the  case 
in  the  earlier  high-temperature  gas-thermometer  measurements.  The  same 
may  be  said  of  the  expansion  of  the  bulb  itself;  a  determination  of  the  ex- 
pansion coefficient,  however  accurate  for  the  range  of  temperatures  from 
zero  to  100°,  or  to  500°,  affords  insufficient  .basis  for  an  extrapolation  to 
1600°,  particularly  since  the  expansion  has  been  definitely  shown  not  to  be 
a  linear  function,  but  to  increase  with  some  rapidity  with  the  temperature  in 
the  case  of  the  platinum  alloys  which  serve  the  purpose  best.  Such  hap- 
hazard extrapolation  appears  particularly  rash  when  it  is  recalled  that  this  is 


SUMMARY.  127 

the  most  important  and  largest  correction  factor  to  which  gas-thermometer 
observations  are  now  subject  and  one  which  can  not  be  avoided  or  materially 
diminished  in  magnitude.  Inasmuch  as  the  bulb  itself  is  practically  in- 
accessible for  the  purpose  of  measuring  its  expansion,  this  difficulty  was  met 
in  the  present  experiments  by  having  a  half-meter  bar  made  up  at  the  same 
time  as  the  bulb  and  from  the  same  material.  The  linear  expansion  of  this 
bar  was  measured  directly  in  a  special  form  of  comparator  (p.  27)  up  to 
1400°;  beyond  this  temperature  the  bar  sagged  under  its  own  weight  and 
accurate  measurements  were  no  longer  practicable.  From  1400°  to  1550°, 
therefore,  we  were  also  obliged  to  resort  to  extrapolation;  but  with  con- 
sistent measurements  (maximum  difference  0.5  per  cent),  several  times 
repeated  at  every  50°  interval  from  300°  to  1400°,  the  extrapolation  over  an 
additional  150°  appears  reasonably  free  from  uncertainty  unless  indeed  the 
alloy  should  be  found  to  possess  an  inversion  point  in  that  region.  But  such 
a  point  would  certainly  show  itself  in  the  form  of  a  break  in  the  series  of  gas- 
thermometer  observations  themselves  and  probably  also  in  the  electromo- 
tive force  of  thermo-elements  of  this  composition,  and  of  such  irregularities 
the  observations  reveal  no  trace. 

By  way  of  conclusion  of  this  effort  to  reduce  uncertainty  to  a  minimum,  a 
final  attempt  was  made  by  the  present  observers  to  ascertain  the  order  of 
magnitude  of  the  aggregate  uncertainty  in  the  results  at  1 100°  by  experi- 
mental means.  A  new  furnace  was  prepared  with  a  very  thick  (i  inch)  wall 
(the  original  furnaces  had  no  wall  inside  the  heating  coil)  and  wholly  differ- 
ent arrangements  for  distributing  the  heat  within  it  (see  Fig.  10  and  de- 
scription, p.  56).  In  this  furnace  all  the  correction  factors  which  result  from 
heat  distribution  and  the  contamination  of  thermo-elements  from  the  heat- 
ing coil  were  radically  altered,  but  the  temperature  measurements  at  the 
copper  melting-point  agreed  with  the  mean  of  all  the  determinations  at  this 
temperature  identically  (1082.6°). 

The  interpretation  of  these  measurements  in  terms  of  the  melting-points 
of  readily  available  substances  encounters  certain  difficulties.  The  melting- 
point  of  pure  salts  is  not  sufficiently  sharp  and  is  somewhat  difficult  of  inter- 
pretation. The  metals  which  have  commonly  been  used  for  the  purpose, 
with  the  possible  exception  of  nickel  and  cobalt,  are,  however,  obtainable 
in  sufficiently  uniform  purity  to  guarantee  an  accuracy  within  i°  at  the 
higher  temperatures. 

No  effort  has  been  made  to  prepare  metals  in  our  own  laboratory  of 
exceptional  purity,  for  the  reason  that  such  metals  would  not  be  available 
for  general  use  and  would  therefore  be  of  little  service.  We  have  accordingly 
adopted  metals  which  are  carried  permanently  in  stock  by  dealers  whose 
names  are  given  in  connection  with  each,  from  whom  the  same  metal  in  a 
nominal  quality  equal  to  that  which  we  used  can  be  readily  obtained.  We 
have  analyzed  these  with  extreme  care  to  show  the  exact  content  of  the 
sample  supplied  us.  We  have  duplicated  the  purchases  ourselves,  and  have 
found  no  errors  greater  than  i°  in  their  melting-point  determinations. 

Another  difficulty  arises  from  the  fact  that  the  melting-points  of  the 
purest  metals  available  for  use  as  constants  in  reproducing  a  high-tem- 
perature scale  (zinc,  silver,  gold,  copper,  and  palladium)  are  distributed  in 
such  a  way  that,  although  they  may  be  located  upon  the  gas-thermometer 


128  HIGH   TEMPERATURE    GAS   THERMOMETRY. 

scale  with  a  probable  error  not  greater  than  0.5° at  low  temperatures  or  ic  at 
high  temperatures,  the  calculation  of  a  similar  curve  passing  through  these 
points  may  not  suffice  to  reproduce  the  scale  with  this  accuracy.  In  the 
region  midway  between  zinc  (418.2°)  and  silver  (960.0°)  the  error  of  inter- 
polation may  amount  to  2°,  and  between  copper  and  palladium  to  5°,  even 
with  metals  of  exceptional  purity.  Extrapolation  is  even  more  uncertain. 
This  can  be  avoided  by  locating  a  sufficient  number  of  intermediate  points 
which  are  equally  trustworthy,  if  such  can  be  found.  We  have  not  been 
fortunate  enough  to  find  points  which  fulfil  these  conditions  quite  as  well 
as  the  metals  named  above,  but  if  the  table  (Table  XVII)  offered  in  the 
text  is  accurately  followed,  there  is  little  danger  of  serious  errors,  even  in 
inexperienced  hands. 

In  order  to  facilitate  as  far  as  possible  the  application  of  these  results  in 
general  practice,  a  typical  thermo-element  curve  has  been  tabulated  in 
small  10°  intervals  throughout  its  entire  length  from  melting  ice  to  melting 
platinum,  together  with  a  diagram  showing  the  character  and  magnitude 
of  the  variation  from  this  curve,  which  may  be  expected  to  appear  in  other 
thermo-elements  of  the  same  nominal  composition  (90  parts  platinum,  10 
parts  rhodium) .  With  a  new  platin-rhodium  thermo-element  of  undoubted 
homogeneity,  but  unknown  constants,  it  is  quite  practicable  with  this  table 
to  prepare  a  curve  of  its  electromotive  force  for  any  temperature  with 
sufficient  accuracy  for  most  purposes  (say  5°  at  low  temperatures  and^  10° 
above  1200°)  from  a  single  determination  in  melting  copper.  If  this  ac- 
curacy is  inadequate,  additional  determinations  of  its  electromotive  force 
at  other  temperatures  of  the  list  below  will  help  to  fix  it  more  closely. 

There  is  no  sure  way  to  guard  against  the  contaminating  influence  of 
metal  vapors  upon  a  thermo-element  in  laboratory  or  industrial  practice, 
although  glazed  porcelain  is  usually  effective.  There  are  very  simp^  and 
rapid  means  of  detecting  contamination  in  an  element  and  determining  its 
distribution,  and  with  a  second  element  at  hand  for  an  occasional  comparison 
there  is  little  of  serious  danger  from  this  cause.  In  any  case,  the  slight  in- 
convenience is  well  worth  while  wherever  considerable  accuracy  is  sought, 
for  there  is  no  other  device  yet  available,  in  the  region  between  1 100°  and 
1600°,  which  is  comparable  with  the  thermo-element  in  sensitiveness  and 
general  practicability. 

In  conclusion,  the  list  of  standard  melting-points  is  given  in  tabular  form, 
together  with  an  estimate  of  the  degree  of  trustworthiness  to  be  accorded 
to  each.  Beside  it  for  convenient  comparison  is  the  present  Reichsanstalt 
scale.  It  may  be  added  that  no  indication  of  a  limit  to  the  temperature 
attainable  with  the  nitrogen  thermometer  or  to  its  ultimate  accuracy  was 
discovered  during  the  present  investigation. 


SUMMARY.  129 

TABLE  XIX. 


Reichs- 

Substance.  Point  Atmosphere.  Crucible  Temperature.         anstalt 

scale. 


j  Zinc  

Antimony. 
Silver  .... 
Gold  
Copper  .  .  . 
Diopside  . 

Mel  ting  and 
freezing 
.  .  ..Do  
....  Do  
....Do  
....  Do  
Melting  

Air  

Carbon  monoxide. 
Do  
.  .    Do  
.  ..  .Do  
Air  

Graphite 

..Do.  .  . 
.  ..Do  
....  Do  
....Do  
Platinum  .  . 

418.2° 

629.2 
.  .      960.0 
.  .    1  062  .  4 
.  .    1082.6 
1391.2 

±0.3 

±0.5 
±0.7 
±0.8 
±0.8 
=•='•5 

419.0 

630.6 

£'  -5 

1064.0 
1084.  i 

(pure) 

Nickel...    Melting  and    Hydrogen  and  ni-  Magnesia    and      1452.3   ±2.0 

freezing.           trogen.  magnesium 
aluminate. 

Cobalt Do Do Magnesia 1489.8   ±2.0    1575 

Palladium    ...  Do Air Pure  magnesia  .     1549.2   ±2.0 

Anorthite.   Melting Do Platinum '549  5   ±2.0 

(pure) 
I 


In  addition,  the  following  temperatures  were  incidentally  obtained: 


Cadmium.   Melting  and  Air Graphite *  320.0° ±0.3     321 .7 

freezing. 

Aluminum    Freezing. . .  .  Carbon  monoxide  .  .  .  .Do 658.0  ±0.6     657. 

Li^SiO.,  .  .    Melting  ....  Air Platinum 1201 .     ±  i . 

Platinum  .    Melting Air 


Holborn  and  Valentiner,  loc.  cit.  '-'Extrapolation,  thermo-electric.  ^Extrapolation,  optical. 

GEOPHYSICAL  LABORATORY, 

CARNEGIE  INSTITUTION  OF  WASHINGTON, 
Washington,  D.  C.,  March,  IQII. 


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